Diagnostics & Surveillance

Every case count, every prevalence estimate, and every genomic tree starts with a laboratory test that says “this sample contains this pathogen.” The methods differ in what they detect, how fast, how cheaply, and with what infrastructure — and no single assay wins on every axis. This collection walks through the workhorse techniques of infectious-disease diagnostics and the trade-offs that decide which one belongs in a reference lab, a clinic, or a field tent.

What each assay class detects across the course of an infection: nucleic acid and antigen appear early and transiently, while IgM and then IgG antibodies rise later and persist.

What each method detects

A diagnostic can target any layer of the pathogen or the host’s response to it, and the target sets both the timing of detection and the meaning of a positive.

Nucleic acid (the pathogen’s RNA/DNA) — qPCR, LAMP. Most sensitive and specific; positive early, during active infection.
Antigen (pathogen proteins) — rapid antigen tests, antigen-capture ELISA. Fast and cheap; positive only when pathogen load is high.
Antibody (the host’s immune response) — ELISA, Western blot. Positive after seroconversion; the basis of serosurveillance and evidence of past exposure.
Whole organism — culture and Gram stain, microscopy. Direct growth or visualization; still the reference standard for many bacteria and parasites.
Molecular fingerprint — MALDI-TOF mass spectrometry, electron microscopy. Identifies an organism from its protein spectrum or its ultrastructure.

The diagnostic window

The figure above is the single most useful idea in the field: timing determines which test is positive. Nucleic acid and antigen appear first, tracking active infection, then fade. Antibodies (IgM, then IgG) appear only after the immune system responds, so a serological test taken too early is falsely negative, while a PCR taken weeks after recovery can be falsely negative even though the person was truly infected. This is why case definitions often pair a molecular confirmatory test for acute infection with serology for past exposure.

Trade-offs: no assay wins on every axis

Choosing a diagnostic is an exercise in constrained optimization across sensitivity, specificity, speed, cost, and the infrastructure and training a method demands.

Diagnostic methods plotted by time-to-result against analytical sensitivity, with bubble size showing cost and complexity: rapid tests are fast but insensitive, PCR is sensitive but slower and costlier, and culture and microscopy occupy their own niches.

Method	Detects	Sensitivity	Specificity	Turnaround	Cost / complexity	Key resource constraint
qPCR	nucleic acid	very high	very high	hours	high (thermocycler, cold chain)	reagents, trained staff, power
LAMP	nucleic acid	high	high	~30–60 min	low–moderate	primer design; contamination control
ELISA	antigen / antibody	moderate–high	high	hours	moderate (plate reader)	antibody reagents, batch controls
Rapid antigen test	antigen	low–moderate	high	~15 min	very low	none — point-of-care
Culture & Gram stain	live organism	high	high	1–5 days	moderate	viable sample, biosafety, skilled tech
Microscopy	organism / cells	variable	moderate–high	minutes	low	expert microscopist
SDS-PAGE / Western	specific protein	moderate	very high	hours–1 day	moderate	antibodies, technical skill
MALDI-TOF	protein fingerprint	high	high	minutes (post-culture)	high capital, low per-test	~US$150–250k instrument; a culture first
$	Electron microscopy	ultrastructure	low	moderate	hours	very high
$
Two lessons recur across the pages that follow.

Sensitivity and specificity are not fixed properties of an assay — they depend on the sampling timing (the diagnostic window), the specimen quality, and the prevalence in the tested population, which drives the positive predictive value.
The best test on paper is often the wrong test in practice. A PCR assay with 99% sensitivity is useless without cold chain, stable power, reagent supply, and trained technologists; a rapid test with 60% sensitivity that returns a result in fifteen minutes at the bedside may avert far more transmission.

From diagnosis to surveillance

Individual tests aggregate into surveillance — the systematic monitoring that tells us where and how fast a pathogen is spreading.

Screening vs confirmatory testing — cheap, fast, sensitive tests screen broadly; specific tests confirm positives, a two-tier strategy that manages the false-positive burden at low prevalence.
Pooled testing — combining specimens multiplies throughput when prevalence is low, at some cost in sensitivity.
Wastewater surveillance — qPCR or sequencing of sewage tracks community transmission without testing individuals.
Genomic surveillance — sequencing positives reveals variants and reconstructs transmission (see the molecular clock and phylodynamics).

Methods

qPCR and RT-qPCR — amplification, Ct values, standard curves, and efficiency
LAMP: Isothermal Amplification — point-of-care molecular testing without a thermocycler
ELISA — plate immunoassays and the four-parameter logistic curve
Rapid Antigen & Lateral-Flow Tests — bedside antigen detection and its prevalence dependence
Culture and the Gram Stain — growing and classifying bacteria, and susceptibility testing
Diagnostic Microscopy and Parasitology — blood films, stains, and crystal analysis
SDS-PAGE and Western Blotting — separating proteins and confirmatory serology
MALDI-TOF Mass Spectrometry — identifying microbes by their protein fingerprint
Electron Microscopy — visualizing virions and ultrastructure

Diagnostic Testing and Screening — sensitivity, specificity, PPV, and ROC
Data Ingestion & APIs — pulling sequence data from GenBank and GISAID
The Molecular Clock and Phylodynamics
Epidemiology