Immunochromatographic rapid tests (lateral flow)

Principle

• The sample flows by capillary action along a strip that contains labelled antibody or antigen and capture lines.
• Strip parts: sample pad, conjugate pad (colloidal gold or coloured latex conjugate), nitrocellulose membrane with test (T) and control (C) lines, absorbent (wick) pad, backing card.
• Sandwich format (antigen detection, e.g. hCG, malaria HRP2/pLDH, SARS-CoV-2 nucleocapsid, HBsAg): analyte binds labelled antibody and is captured at the test line; a coloured line means positive.
• Competitive format (small molecules, e.g. drugs of abuse): analyte blocks binding at the test line; absence of a test line means positive.
• Antibody detection (e.g. HIV, HCV, syphilis): antigen on the test line captures patient antibody, detected by a labelled reagent.
• The control line captures excess conjugate and confirms correct flow.

Specimen

• Serum, plasma, whole blood (fingerprick), urine, nasal or nasopharyngeal swabs, stool, depending on the kit.

Procedure

• Bring kit to room temperature; add the exact sample volume and buffer drops stated.
• Read within the stated window (commonly 10–20 minutes); do not read after the maximum time.

Results

• Valid only if the control line appears.
• Sandwich: C + T lines = positive (any visible T line, even faint); C only = negative.
• No C line = invalid; repeat with a new device.

Quality control and pitfalls

• Run kit or external controls with each new lot, shipment or operator; store within temperature and humidity limits; check expiry.
• High-dose hook effect (e.g. very high hCG) can give false negatives; dilute and retest.
• P. falciparum hrp2/hrp3 deletions cause false negative HRP2 tests; HRP2 persists for weeks after treatment.
• Generally less sensitive than laboratory methods; confirm by reference tests according to national algorithms.

Clinical use

• Point-of-care testing for pregnancy, malaria, HIV, HBV, HCV, syphilis, dengue NS1/IgM, COVID-19, troponin and drugs of abuse; WHO REASSURED criteria describe ideal characteristics.