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Assays & Diagnostics: 161 3D models

ELISA, blots, flow cytometry, PCR, sequencing, cell assays and rapid tests, step by step (real sizes µL → bench). Each one is a ready-made 3D model you can turn, recolour and place in a figure — open BioDiorama and type its name in the Library search.

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ELISA & immunoassays · Western blot & electrophoresis · Flow cytometry & cell sorting · PCR & amplification · Arrays & sequencing · Imaging & tissue staining · Cell-based assays · Rapid tests & diagnostics · Heart disease — diagnosis · Infection — making the diagnosis

ELISA & immunoassays 21

ELISA well (cut-away) — One microplate well cut open to show the layers that build up: capture antibody, blocking protein, antigen, detection antibody, enzyme and the coloured product. Choose the ELISA format and the step. (well ≈ 7 mm across (molecules enlarged))
ELISA steps (six wells) — Six cut-away wells in a row, one per step: coat, block, bind, detect, substrate and stop. (wells ≈ 7 mm (molecules enlarged))
ELISA strip plate (colour developed) — A 96-well ELISA plate of 8-well breakaway strips: a two-fold standard series in columns 1–2 and samples across the plate, coloured by how much antigen each held. One strip lifts out. (128 × 86 mm)
Digital ELISA (single-molecule bead array) — Beads carrying single captured molecules settle into femtolitre wells; wells holding an enzyme light up and are simply counted. (wells ≈ 4 µm (enlarged))
Chemiluminescent immunoassay bead — A paramagnetic microparticle coated with capture antibodies; sandwiches carrying a chemiluminescent label flash light when the trigger solution is added. (bead ≈ 1–3 µm (molecules enlarged))
Multi-spot electrochemiluminescence well — A black well with a carbon-electrode floor printed with ten capture spots; each spot glows in proportion to its analyte when voltage is applied. (well ≈ 7 mm)
Multiplex bead immunoassay — Colour-coded beads, each carrying a different capture antibody; one laser reads the bead colour and another the reporter on the bound sandwich. (beads ≈ 6 µm (molecules enlarged))
Proximity bead assay (AlphaLISA) — A donor and an acceptor bead brought together by the analyte: red light makes the donor release singlet oxygen, which reaches only a nearby acceptor and makes it glow. (beads ≈ 200 nm (enlarged))
TR-FRET immunoassay pair (HTRF) — Two antibodies on one analyte: a europium donor and an acceptor dye close enough for energy transfer, read after a time delay. (≈ 10–20 nm (schematic))
Magnetic bead separation — Beads (for immunoprecipitation or DNA clean-up) pulled out of solution onto the tube wall by a magnet, leaving a clear supernatant to pipette off. (1.5 mL tube)
Dot blot — Samples spotted straight onto a nitrocellulose membrane and probed with antibody: darker dots hold more target. (≈ 7 × 5 cm)
Antibody array slide (16 pads) — A glass slide with 16 nitrocellulose pads, each printed with a grid of capture antibodies; bound proteins light up as fluorescent spots. (75 × 25 mm slide)
Agglutination card test — Drops of sample and latex (or charcoal) reagent mixed in printed circles: positive circles clump within minutes, negative ones stay smooth. (≈ 10 × 6 cm)
Haemagglutination plate — Red cells settle into a tight button unless virus (or antibody) links them into a mat; each row is a two-fold dilution series and the titre is the last mat. (128 × 86 mm)
Ouchterlony double immunodiffusion — Antiserum in the centre well and antigens around it diffuse through agarose; white precipitin lines form where they meet, showing identity, partial identity or non-identity. (⌀ 9 cm dish)
Radial immunodiffusion plate — Antigen diffuses from each well into agarose that holds antibody; the precipitin ring grows with antigen concentration. (≈ 8 × 6 cm)
Immunoelectrophoresis — Proteins separated by electrophoresis in agarose, then antiserum in the trough forms precipitin arcs; or Laurell rocket electrophoresis, where rocket height measures concentration. (75 × 25 mm slide)
Tube agglutination and ring tests — Glass tubes read by eye: red-cell agglutination graded 0 to 4+ after centrifuging, or a white precipitin ring where antigen meets antiserum. (12 × 75 mm tubes)
Biolayer interferometry sensor tip — The end of a fibre-optic biosensor dipped in a well: as analyte binds the ligand layer, the light reflected from the two interfaces shifts. (tip ⌀ ≈ 1 mm (molecules enlarged))
Line immunoassay strips — Strips printed with many antigens as lines, incubated with patient serum in a tray: each purple band marks antibodies to one antigen. (strips ≈ 9 cm)
Protein assay standards (cuvettes) — A row of cuvettes with a protein standard series: the colour deepens with concentration (Bradford, BCA or biuret). (≈ 12 cm)

Western blot & electrophoresis 12

Polyacrylamide gel (after the run) — A mini protein gel freed from its glass plates: stacking gel with wells, a ladder lane and sample lanes. Pick the stain: Coomassie, silver, unstained with a prestained ladder, zymography or an EMSA autoradiograph. (8.3 × 6.8 cm)
Gel cassette: casting and loading — Two glass plates and spacers holding a mini gel: casting under an overlay, the comb in place, samples loaded with a gel-loading tip, or running. (10 × 8 cm)
Wet transfer cassette (opened) — The tank-transfer sandwich laid out on the open cassette: fibre pad, filter paper, gel, membrane, filter paper, fibre pad, black side to the cathode. (≈ 11 × 10 cm)
Western blot membrane — A blot after transfer, with its cut corner and pencilled ladder: Ponceau S total-protein stain, colorimetric bands, or a two-colour fluorescent scan. (≈ 8 × 6 cm)
Chemiluminescence film and cassette — An open film cassette with the blot in a sheet protector, and the developed X-ray film beside it showing dark target and loading-control bands. (≈ 15 × 12 cm)
Antibody detection on a blot (close-up) — Inside a band on the membrane: target protein held on the fibres, primary antibody, enzyme- or dye-labelled secondary, and the light they give. (≈ 100 nm (schematic))
2-D gel — Proteins separated first by charge (isoelectric point, across) and then by size (down), each a spot; or a DIGE overlay of two samples. (≈ 9 × 7 cm)
Isoelectric focusing strip — An immobilised pH-gradient strip in its focusing tray between electrode wicks; proteins stop where their charge is zero. (13 cm strip)
Southern / northern blot (capillary transfer) — The classic capillary stack: buffer wicks up through the gel and carries DNA or RNA onto a nylon membrane, drawn by a pile of paper towels under a weight. (≈ 13 × 10 cm)
Cutting a band out of an agarose gel — A DNA gel on a blue-light transilluminator, one band cut out with a scalpel and dropped into a tube for gel extraction. (≈ 12 × 10 cm)
Hybridisation bottle with blot — A glass roller bottle with the membrane curled against its wall and a little labelled probe; it turns slowly in the hybridisation oven. (⌀ 3.5 × 15 cm)
Western blot workflow — The four stages in a row: separate proteins on a gel, transfer them to a membrane, probe with antibodies, and detect the bands. (≈ 30 cm (schematic))

Flow cytometry & cell sorting 9

Hydrodynamic focusing (flow cell) — Sheath fluid squeezes the sample stream down to a narrow core so that cells pass the laser one at a time. (channel ≈ 200 µm)
Cell crossing the laser beams — A stained cell in the stream at the laser beams: forward scatter (size), side scatter (granularity) and fluorescence from its labelled antibodies. (cell ≈ 10 µm)
Droplet cell sorting — The jet breaks into droplets; droplets holding a wanted cell are charged and deflected between high-voltage plates into collection tubes. (≈ 5 cm)
FACS tube with cell-strainer cap — A 5 mL round-bottom tube of stained cells, filtered through the 35 µm mesh in its cap before running on the cytometer. (12 × 75 mm)
Antibody-stained cell (immunophenotyping) — A lymphocyte whose surface markers carry fluorochrome-labelled antibodies in up to four colours. (cell ≈ 8 µm (antibodies enlarged))
Spectral detector array — Light from the cell is spread into a spectrum by a prism and read by a row of detectors, giving each fluorochrome its own signature. (≈ 10 cm)
Microfluidic cell sorter chip — A chip-based sorter: sheath streams focus the cells, a laser reads them, and a pulse pushes chosen cells into the sort channel. (≈ 9 × 5 cm)
Imaging flow cytometry — Cells imaged as they flow past an objective: each cell gives a row of brightfield, fluorescence and scatter pictures. (≈ 1 mm (images enlarged))
Mass cytometry (metal-tagged cell in plasma) — A cell labelled with metal-tagged antibodies is vaporised in an argon plasma; the metal ions are counted by time-of-flight. (≈ 1 cm (cell enlarged))

PCR & amplification 17

PCR tube with master mix (cut-away) — A 0.2 mL PCR tube cut open to show what the master mix holds: template DNA, primers, the four dNTPs, Taq polymerase and magnesium. (tube ≈ 2 cm (molecules enlarged))
DNA melting, annealing and extension — A double helix through one PCR cycle: separated at 95 °C, primers annealing as it cools, and polymerase extending the new strands at 72 °C. (≈ 4 nm (schematic))
Hydrolysis (TaqMan) probe — A probe with a reporter dye and a quencher sits on the template; the polymerase cuts it as it extends, freeing the reporter to fluoresce. (≈ 10 nm (schematic))
Intercalating dye on DNA (SYBR-type) — Dye molecules light up only when bound in double-stranded DNA, so the signal rises as PCR product accumulates. (≈ 4 nm (schematic))
qPCR plate (fluorescing wells) — A sealed 96-well PCR plate partway through a real-time run: wells with more template cross the threshold earlier and glow brighter. (128 × 86 mm)
Droplet digital PCR emulsion — A well of thousands of identical droplets after PCR: those that held a target molecule glow, and counting them gives the absolute copy number. (droplets ≈ 100 µm (enlarged))
Droplet reading channel — Droplets pass one by one through a detector, spaced apart by oil, and each is called positive or negative. (channel ≈ 100 µm)
Digital PCR partition chip — A chip with thousands of fixed micro-chambers per sample; after cycling, the lit chambers are counted. (≈ 12 × 7 cm)
LAMP colour-change tubes — Isothermal LAMP reactions read by eye: phenol red turns from pink to yellow, HNB from violet to sky blue, or calcein glows green under UV. (0.2 mL tubes)
LAMP dumbbell DNA — The looped "dumbbell" and cauliflower-like repeats that loop-mediated amplification builds from its inner and loop primers. (≈ 20 nm (schematic))
Rolling-circle amplification — A polymerase runs round a small circular template again and again, spinning off a long single strand of repeats. (≈ 50 nm (schematic))
Emulsion PCR bead — One bead in one oil droplet: a single starting molecule is amplified into thousands of identical copies on the bead for sequencing. (droplet ≈ 10 µm (schematic))
Continuous-flow PCR chip — Sample plugs flow along a serpentine channel that crosses three heated zones, cycling as they go. (≈ 10 × 6 cm)
Reverse transcription — Reverse transcriptase copies mRNA into cDNA from an oligo(dT) primer on the poly(A) tail — the first step of RT-PCR. (≈ 10 nm (schematic))
PCR set-up on ice — An ice bucket with a PCR tube strip and the master-mix, primer and template tubes kept cold while setting up reactions. (≈ 12 × 9 cm)
Spin column (nucleic-acid purification) — A silica-membrane column in its collection tube, cut open: DNA binds the membrane while buffers spin through. (≈ 4.5 cm)
DNA precipitated in ethanol — White strands of DNA rising into a cold ethanol layer and winding onto a glass rod. (⌀ 3.5 cm tube)

Arrays & sequencing 12

Two-colour microarray slide — A glass slide printed with blocks of DNA spots after hybridising two labelled samples: red, green or yellow shows where each gene is more active. (75 × 25 mm)
Microarray spot (close-up) — One spot on the array: probe strands fixed to the glass, some paired with dye-labelled target cDNA. (spot ≈ 100 µm (molecules enlarged))
Oligonucleotide array cartridge — A sealed plastic cartridge with a glass chip of millions of probe features, filled through rubber septa. (≈ 6 × 4.5 cm)
Sequencing clusters on a flow cell (close-up) — The lane surface during a run: each cluster of identical DNA copies lights up in the colour of the base it is reading. (clusters ≈ 1 µm (enlarged))
Bridge amplification — Library strands bend over to bind surface oligos and are copied in place, building a cluster of identical molecules. (≈ 1 µm (schematic))
Sequencing by synthesis — A polymerase adds one dye-labelled, reversibly blocked nucleotide per cycle; the colour is imaged, then the dye and block are removed. (≈ 10 nm (schematic))
DNA strand through a nanopore — A motor protein unzips DNA and feeds one strand through a protein pore in a membrane; the ion current changes with the bases inside. (≈ 10 nm (schematic))
Zero-mode waveguide wells — Tiny holes in a metal film, each with one polymerase at the bottom; only nucleotides being added there flash in the light from below. (wells ≈ 100 nm (schematic))
Microwell sequencing (semiconductor / pyro) — A bead of clonal DNA in a microwell: each added base releases a proton read by the sensor below, or a flash of light in pyrosequencing. (well ≈ 3 µm (schematic))
Sanger sequencing capillary — Dye-terminated DNA fragments separate by length in a capillary and pass a laser one by one, each glowing in its base colour. (capillary ⌀ 50 µm (enlarged))
Chromatin immunoprecipitation (ChIP) — Sheared chromatin with a DNA-binding protein crosslinked in place, captured by an antibody on a protein A/G bead. (≈ 50 nm (schematic))
Sequencing library fragment — One library molecule: your DNA insert between read-primer sites, index barcodes and the flow-cell adapters. (≈ 100 nm (schematic))

Imaging & tissue staining 18

Immunofluorescent cells on a coverslip (close-up) — Fixed cells spread on glass, stained for nuclei (blue), actin filaments (red) and a third target (green). (cells ≈ 30 µm)
Stained tissue slide — A labelled glass slide with a thin tissue section under a coverslip: immunohistochemistry (brown DAB), H&E, trichrome or PAS. (75 × 25 mm)
IHC staining in tissue (close-up) — Cells in a section with brown DAB marking the target: nuclear (e.g. Ki-67), membrane (e.g. HER2) or cytoplasmic, with blue haematoxylin nuclei. (≈ 100 µm)
Metaphase chromosome spread (FISH) — Chromosomes from one dividing cell spread on a slide and counterstained blue, with fluorescent probes marking chosen loci; or every chromosome painted its own colour (M-FISH). (spread ≈ 50 µm)
Interphase FISH nuclei — Nuclei with probe signals as dots: a normal pattern, a gene fusion (yellow), a break-apart split, or an amplification cluster. (nuclei ≈ 10 µm)
G-banded karyogram — The chromosomes of one cell paired and lined up from 1 to 22 plus X and Y, each with its light and dark G-bands. (chromosomes ≈ 2–10 µm (enlarged))
Tissue microarray slide — Dozens of small tissue cores from different patients in a grid on one slide, stained together. (75 × 25 mm)
Paraffin block in a cassette — Tissue embedded in paraffin wax on a labelled processing cassette, its face trimmed ready for the microtome. (≈ 4 × 3 cm)
Section ribbon on a water bath — A ribbon of paraffin sections floating flat on warm water, one being picked up on a slide. (⌀ 14 cm bath)
Cleared mouse brain in a vial — A whole mouse brain made transparent by tissue clearing, with fluorescently labelled neurons inside, ready for light-sheet imaging. (≈ 1.5 cm brain)
Expansion microscopy gel — Cells anchored in a swellable gel before and after soaking in water: the gel grows about four times larger, spreading the labels apart. (⌀ 9 cm dish)
Single-molecule dots in a cell (PLA, smFISH) — A cell with countable dots: proximity-ligation signals where two proteins meet, single mRNA molecules, or chromogenic RNA probes. (cell ≈ 30 µm)
Live-cell imaging dish — A 35 mm dish with a glass coverslip floor for high-resolution imaging, cells in medium above an inverted objective. (⌀ 35 mm)
Staining jars (staining series) — A row of Coplin jars for staining slides by hand: dewax, rehydrate, stain and dehydrate. (jars ≈ 9 cm)
Frozen tissue block (OCT) — Tissue frozen in OCT compound on a cryostat chuck, with an empty cryomold beside it. (≈ 2 cm block)
Zebrafish larvae mounted in agarose — Larvae held in low-melt agarose on a glass-bottom dish, ready for live imaging. (⌀ 35 mm dish)
Cytospin slide preparation — A cell suspension in a funnel chamber clipped to a slide with a filter card; centrifuging lays the cells in one round spot. (≈ 10 cm)
Immunostaining steps on a cell — The same cell through fixation, permeabilisation, blocking, primary antibody and fluorescent secondary antibody. (cells ≈ 20 µm (schematic))

Cell-based assays 27

Cell viability plate (dose response) — A 96-well plate after a colorimetric viability read: colour falls as drug dose rises across the plate (untreated control on the right). (128 × 86 mm)
MTT formazan crystals in cells — Living cells reduce yellow MTT to purple formazan, which builds up as needle-like crystals before it is dissolved and read. (cells ≈ 30 µm)
Live/dead staining — Live cells glow green with calcein; dead cells, rounded and leaky, show red-stained nuclei. (field ≈ 300 µm)
Luminescence plate (luciferase) — A white opaque plate in which wells glow with luciferase light — reporter-gene, ATP or caspase luminescence assays. (128 × 86 mm)
ELISpot wells — Wells with PVDF floors after an ELISpot: each dark spot marks one cell that secreted cytokine. (wells ⌀ ≈ 6 mm)
ELISpot: a cell making its spot (close-up) — A T cell on the membrane releases cytokine, which is caught by antibodies around it and developed into a coloured spot. (≈ 20 µm (molecules enlarged))
Soft agar colony assay — Cells in a soft top agar over a firmer base layer: only transformed cells grow into colonies without anchorage. (well ⌀ ≈ 35 mm)
Spheroid invasion assay — A tumour spheroid embedded in collagen, sending invasive strands of cells out into the matrix. (≈ 500 µm)
Hanging-drop spheroids — Drops of cell suspension hanging from an inverted lid; cells gather at the tip of each drop into a spheroid. (⌀ 10 cm dish)
Cells crossing a porous membrane (close-up) — A section through a transwell membrane: cells squeeze through 8 µm pores towards a chemoattractant; or a monolayer barrier tested with a tracer. (≈ 60 µm)
Apoptotic cell (annexin V / PI) — An apoptotic cell with blebs and phosphatidylserine flipped outward, stained green by annexin V; late cells also let in red propidium iodide. (cell ≈ 15 µm)
Phagocytosis assay — A macrophage that has engulfed pH-sensitive particles: they light up only inside acidic phagolysosomes. (cell ≈ 30 µm)
Impedance (real-time cell analysis) well — Cells growing over gold interdigitated electrodes in the well floor; the more they cover, the higher the impedance. (well ⌀ ≈ 5 mm)
Micronucleus assay cell — A binucleated cell (division blocked after nuclear division) with a small extra micronucleus — a sign of chromosome damage. (cell ≈ 25 µm)
Haemolysis assay tubes — Red cells incubated with rising doses (or falling salt): intact cells settle as a pellet, lysed cells turn the supernatant red. (12 × 75 mm tubes)
Aortic ring sprouting assay — A ring of mouse aorta in gel sends out new microvessel sprouts — an ex vivo angiogenesis assay. (ring ≈ 1 mm)
Neurite outgrowth assay — Neurons stained for β-III-tubulin, extending branching neurites with growth cones; neurite length is the read-out. (field ≈ 300 µm)
Culture-insert gap (wound) assay — A silicone two-well insert makes a clean cell-free gap: in place while cells grow, then lifted off so they can migrate in. (⌀ 35 mm dish)
LDH release (cytotoxicity) — A damaged cell leaks lactate dehydrogenase into the medium, where it drives a red formazan colour. (cell ≈ 15 µm)
Chemotaxis slide — A slide with two reservoirs joined by a thin channel where a stable gradient forms; cells migrate up it under the microscope. (75 × 25 mm)
Yeast spot assay plate — Ten-fold dilutions of yeast strains spotted in rows: growth shows sensitivity; on dropout plates it shows two-hybrid interactions. (⌀ 9 cm dish)
Blue–white screening plate — Bacterial colonies after cloning: white colonies carry an insert that disrupts lacZ, blue ones do not. (⌀ 9 cm dish)
Replica plating — Colonies pressed onto sterile velvet and printed onto a selective plate: those missing there cannot grow on it. (≈ 28 cm across)
CAMP test plate — Group B streptococcus streaked at right angles to S. aureus on blood agar: a positive gives an arrowhead of enhanced haemolysis. (⌀ 9 cm dish)
Catalase and oxidase tests — Two quick bench tests: colony material bubbles in hydrogen peroxide (catalase), or turns dark purple on oxidase reagent. (≈ 7 cm)
Trypan blue count (counting grid) — Cells over the counting grid of a haemocytometer: live cells exclude the dye and look bright, dead cells stain blue. (1 mm squares)
Peg-lid biofilm device — A lid of 96 pegs grown with biofilm, lifted from a plate of antibiotic dilutions; stained biofilm shows which pegs survived. (128 × 86 mm)

Rapid tests & diagnostics 27

Lateral-flow test cassette (opened) — A rapid-test cassette with its cover lifted off, showing the strip inside: sample pad, gold-conjugate pad, membrane with test and control lines, wicking pad. (≈ 7.5 × 2 cm)
Lateral-flow dipstick in a sample tube — A bare test strip standing in a tube of sample; liquid climbs the membrane and the lines appear. (≈ 7 cm strip)
Midstream pregnancy test — A midstream hCG test with absorbent tip and cap: two lines (or a digital read-out) for positive. (≈ 15 cm)
Rapid antigen test kit (laid out) — Everything in a self-test box: cassette, nasal swab, extraction tube with dropper cap in its stand, and the foil pouch. (≈ 12 cm)
Multiplex rapid test (three targets) — One cassette that tests for several targets at once, with a test line for each and one control line. (≈ 8 × 2.5 cm)
Urine drug-screen cup — A specimen cup with built-in test strips for several drugs and a temperature strip; here a missing test line means positive. (≈ 6 × 7 cm)
Blood glucose test strip (close-up) — An electrochemical test strip: a fingertip drop is drawn into the capillary channel over the enzyme layer and electrodes. (≈ 3.5 cm strip)
Urine dipstick and colour chart — A ten-pad reagent strip next to its bottle, whose label carries the colour scale for each pad. (≈ 11 cm strip)
Blood-typing card — A bedside typing card: anti-A, anti-B and anti-D circles plus a control; agglutination shows which antigens the red cells carry. (≈ 10 × 5 cm)
Column agglutination gel card — A gel card of six microtubes after centrifuging: agglutinated cells stay at the top of the gel, free cells pellet at the bottom. (≈ 5 × 7 cm)
Heel-prick (dried blood spot) card — A newborn screening card: drops of heel-prick blood dried on printed filter-paper circles, with the form above. (≈ 13 × 8 cm)
Point-of-care test cartridge — A single-use microfluidic cartridge for a bedside analyser: blood chemistry sensors, or reagent blisters and amplification wells for a molecular panel. (≈ 5 × 9 cm)
ESR tubes (Westergren) — Tall tubes of anticoagulated blood in a stand: red cells settle, and the clear plasma above after one hour is the sedimentation rate. (≈ 20 cm tubes)
Haematocrit capillary tubes — Spun capillary tubes sealed with clay: packed red cells, a thin buffy coat and plasma; the red fraction is the haematocrit. (≈ 7.5 cm tubes)
Stool blood test (card or FIT tube) — A guaiac card turning blue with hidden blood, or a faecal immunochemical test tube with its grooved sampling probe. (≈ 8 cm)
Urine dipslide — A paddle coated with two agars, dipped in urine and incubated in its tube; colony density is compared with a chart. (≈ 11 cm)
Paper microfluidic test — A wax-patterned paper device: a drop in the centre wicks along channels to test zones that change colour. (≈ 8 × 8 cm)
Saliva collection tube — A tube with a funnel for spitting saliva up to the fill line, then capped with a stabilising buffer (DNA or PCR testing). (≈ 10 cm)
Oral-fluid self-test — A flat pad swabbed along the gums, then stood in its developer vial; lines appear in the window. (≈ 12 cm)
Spun serum tubes (sample quality) — Centrifuged serum tubes with clot, separator gel and serum: normal, haemolysed, icteric and lipaemic samples side by side. (13 × 100 mm tubes)
Cervical screening vial and brush — A liquid-based cytology vial of preservative and the broom-type sampler used to collect cells (Pap / HPV testing). (≈ 8 cm vial)
Smartphone reading a rapid test — A phone camera reading a lateral-flow cassette and showing the result in an app. (≈ 15 cm)
Making a blood film — A spreader slide pushing a drop of blood into a thin film with a feathered edge; or thick and thin films on one slide (malaria). (75 × 25 mm slide)
Capillary blood collection — A fingertip drop after a safety lancet, collected into a small microtube with a scoop cap. (≈ 8 cm)
Skin tests on the forearm — Allergy skin-prick test with wheals and flare at positive sites, or a tuberculin (Mantoux) induration measured with a ruler. (≈ 20 cm)
Specimen transport bag — A sealed biohazard bag with blood tubes in the main pouch and the request form in its own pocket. (≈ 23 × 30 cm)
Urine sediment under the microscope — A field of urine sediment: red and white cells, casts, oxalate and phosphate crystals, and a squamous cell. (field ≈ 200 µm)

Heart disease — diagnosis 10

The ECG, and six things it can show — The trace itself rather than the machine, on paper at twenty-five millimetres a second. P wave for the atria, QRS for the ventricles — under a hundred and twenty milliseconds if the wiring is intact — an ST segment that should sit on the baseline, and a T wave as they recover. Then the findings: ST elevation, which means an artery is blocked now; ST depression and T inversion, which means ischaemia; Q waves, which mean muscle that died some time ago; a broad notched QRS; and atrial fibrillation, with no P waves and no two beats the same distance apart. (one small square = 0.04 s and 0.1 mV)
The echocardiogram — The parasternal long-axis view: the probe between two ribs, the wedge of ultrasound it sees — everything outside which is invisible — and the four things that view is for. Then the findings, each with the measurement it is graded on: a narrow fast jet through a calcified aortic valve, which the Doppler turns into a velocity and a gradient; a jet going backwards into the atrium, graded on its narrowest part and not how far it reaches; or a ventricle that barely moves. (sector reaches 15–20 cm)
The coronary angiogram — What a cath lab produces: the tree filled with contrast and one place where it is not. The narrowing is read as a percentage of the diameter of the healthy segment next to it, which is why the reference matters as much as the lesion. A tight old lesion will have collaterals filling it from the other side. The thing worth remembering is what the picture does not contain: the wall, the cap, the plaque — only the lumen. (left main 3–5 mm)
The coronary calcium score — A non-contrast slice where calcium is bright and nothing else is, scored as area times density added over the whole tree. What makes it unusual is the strength of a negative: a score of zero is the best evidence of low risk in cardiovascular prevention, good for years. Above four hundred the risk is as high as having already had an event. And what it misses is on the same picture — soft plaque with no calcium in it at all. (specks 1–5 mm)
Late gadolinium enhancement — The most diagnostic picture in cardiology, and it works on one principle: the contrast stays where there is more space between the cells than there should be. Where it stays then names the disease. Bright at the inner surface in one artery's territory is an infarct and nothing else does that. Bright in the middle of the wall, sparing both surfaces, is fibrosis. Bright all round the inner half with blood and muscle hard to tell apart is amyloid. Bright under the epicardium is myocarditis. (slice 6–8 mm thick)
The troponin assay, and its curve — Two things that get confused. The assay is a sandwich — an antibody on a bead catches the cardiac isoform, a labelled second antibody completes it, and the label is counted — and it is now sensitive enough to detect troponin in most healthy people. Which is why the single number is almost never the answer: a rise and a fall over hours is an infarct, a level that is up and flat has been like that for months and means prognosis rather than an artery to open, and a very low value two hours after the pain rules it out. (detects below 5 ng/L)
Imaging from inside the artery — The only way to see the wall rather than the lumen, and the two methods answer different questions. Light gives about fifteen micrometres of resolution and separates the three layers, so it is the only test that can measure a fibrous cap — but the blood has to be flushed out first. Sound gives about a hundred and fifty micrometres and sees twice as deep, through blood, which makes it the one for plaque burden and for calcium with its clean shadow behind it. (image 6–10 mm across)
The ankle–brachial index — Four pressures and a division, and still the best bedside test for arterial disease there is. The higher ankle pressure over the higher arm pressure: below 0.9 is disease, 0.5 to 0.9 is where claudication lives, and below 0.5 is critical ischaemia. The trap is at the other end — above 1.3 means the vessel is too stiff to compress and the number means nothing, which is common in diabetes. If it is normal at rest and the story fits, measure it again after walking. (normal 1.0–1.3)
Stress and rest perfusion — The test only says anything because there are two pictures. Under stress a narrowed artery cannot increase its flow the way a normal one can, so a patch takes up less tracer. The question is then what the rest picture shows: if it fills in, that muscle is alive and short of supply, and opening the artery helps. If the defect is in both, that muscle is already dead, and opening anything gains nothing. One picture alone cannot tell the two apart. (short-axis slices)
The electroanatomic map — What an ablation is planned on. A catheter is walked round the inside of the chamber, and because it knows where it is to within a millimetre, the signal at each point can be painted onto a shell of the chamber's real shape. Healthy muscle gives a good signal, scar gives almost none, and a narrow channel of slow tissue through the scar is what the circuit uses — so that channel is the target, together with the ring of lesions round the pulmonary veins. (atrium 4–6 cm across)

Infection — making the diagnosis 8

Why the Gram stain separates anything at all — The oldest test in microbiology, and it works on one physical difference. Crystal violet and iodine make a large complex inside every cell. Then alcohol: it dissolves the outer membrane of a Gram-negative and the complex washes straight out of its one or two layers of wall, while the twenty or thirty layers of a Gram-positive hold it — and dehydrate and close further as the alcohol goes through. The decolourising step is the one that goes wrong: a few seconds too long and everything is Gram-negative. An old or dying Gram-positive loses it anyway, which is why the result is read with the morphology and not alone. (wall 20–80 nm vs 2–7 nm)
Acid-fast: a stain for a wall made of lipid — Mycobacteria need their own stain because their wall is half lipid by weight, with mycolic acids sixty to ninety carbons long. Carbol fuchsin driven in with heat or a detergent dissolves into that lipid, and then acid and alcohol together — which would strip any ordinary organism — do not reach it there. So a red rod on a blue field is the diagnosis and the organisms can be counted. The limitation is severe: it takes about ten thousand organisms per millilitre to see anything, which is why a negative smear means very little. (needs ≈ 10⁴ organisms/mL)
What a CSF result actually says — Four numbers, and together they name the cause before anything is grown. Bacterial: hundreds or thousands of cells, nearly all neutrophils, a high protein, and a glucose that has fallen because something in the fluid is eating it. Viral: tens to hundreds of lymphocytes, protein mildly raised, glucose normal — and that glucose is the single most useful number on the sheet. Tuberculous: lymphocytes, a very high protein, a low glucose, and an illness over days rather than hours. None of it waits for the antibiotic: the fluid is taken and the drug is given, in that order and within the hour. (normal: < 5 cells, 0.45 g/L, glucose 60 % of blood)
What "resistant" on a report means — Not what most people assume. Plot the MIC of every isolate and two populations appear: the wild type, clustered tightly, and a second group that has acquired a mechanism. The line between them is the epidemiological cut-off, and it is a statement about the organism. The clinical breakpoint is a different line, set from what the drug actually reaches at the dose that is given — so an isolate can be non-wild-type and still treatable. That is why a report says susceptible or resistant rather than giving a number, and why the pharmacology differs by class: a beta-lactam needs time above the line, an aminoglycoside needs a high peak. (MICs double at each step)
A Ct value, and a faint line — Two results that are routinely over-read. In an amplification test, the cycle at which the signal crosses the threshold is the Ct, and it is a logarithm — about three cycles to a tenfold difference in how much template was there. It says how much nucleic acid is present, not whether anything is alive: a dead organism is still detected for weeks after treatment. On a lateral flow strip, a faint test line is a positive; how faint depends on how much antigen was on the swab, which depends on the day of illness and on how the swab was taken. And a negative does not rule out, because these need a good deal of antigen to show anything. (Ct 15–40; three cycles ≈ 10-fold)
Lactate and procalcitonin — Two numbers that are routinely over-interpreted. Lactate rises partly because tissue is short of oxygen and partly because adrenaline drives glycolysis, so a raised value is not proof of hypoperfusion on its own — and what predicts survival is the clearance over the next hours, not the first reading. Procalcitonin rises within hours in a bacterial infection, falls fast once it is controlled, and barely moves in a viral one; its best established use is not starting antibiotics but stopping them. Neither number diagnoses sepsis, and neither replaces looking at the patient. (lactate threshold 2 mmol/L)
Is this one outbreak or four coincidences? — The question an outbreak investigation actually asks, and sequencing is the only test that answers it. An organism accumulates single-base changes at a known rate, so a tree built from whole genomes puts a number on relatedness. Nought to two differences between isolates taken weeks apart is far too close to be chance: these came from one source, and the remaining question is which — a sink, a scope, a member of staff, a batch of fluid. Nine or twenty differences means unrelated strains that happened to be on the same ward. The older typing methods could say the same species, not the same strain. (typically 0–5 SNPs within an outbreak)
What a tuberculosis blood test can and cannot say — Three wells: one with nothing, one with antigens that tuberculosis has and the vaccine does not — so a vaccinated person does not react — and one with a general stimulant to show the cells can respond at all. If the patient's T cells have met the organism they release interferon gamma into the middle well and it is measured. The crucial limitation is that it cannot tell latent infection from active disease: a third of the world would test positive and almost none of them are ill. It is a test for deciding whether to treat latent infection, not for diagnosing the person in front of you with a cough. (three wells; result in 24 h)