I performed a Wondfo HIV‑1/2 third‑generation rapid immunochromatographic test on finger‑stick whole blood, which showed only a control line at the 10‑15‑minute reading window (negative). After rinsing the cassette with tap water, a test line appeared. How should this result be interpreted?

Interpretation of HIV Rapid Test After Water Contamination

The test result should be considered invalid and the patient should undergo repeat testing with a new test kit. The appearance of a test line after washing the cassette with tap water represents a procedural error that invalidates the result, not a true positive finding.

Why This Result is Invalid

Rapid HIV tests must be read strictly within the manufacturer's specified time window (10-15 minutes for this device) and any manipulation of the test cassette after this period invalidates the result. 1

• The negative result observed during the proper 10-15 minute reading window is the only valid result from this test. 1
• Washing the test cassette with tap water after the reading window introduces multiple sources of error:
  • Water can cause non-specific binding and background reactions on the immunochromatographic membrane 2
  • Dilution effects may alter the concentration of reagents and cause false line appearance 3
  • Physical disruption of the test membrane can create artifacts that mimic test lines 2

Understanding Rapid Test Technology

Third-generation rapid tests like the Wondfo HIV-1/2 use lateral flow immunochromatography, which relies on precise reagent concentrations and controlled flow conditions. 4, 5

• These tests are designed to be read only during the specified time window because reactions continue to develop after this period, potentially creating false-positive appearances. 1, 3
• The control line (C) appearing during the valid reading window confirms the test functioned properly up to that point. 3
• Any result appearing outside the reading window or after test manipulation has no diagnostic validity. 1

Appropriate Next Steps

The patient requires repeat testing with a new rapid test kit, performed correctly without any post-test manipulation. 1

• Use a fresh test cassette and follow manufacturer instructions precisely, reading results only between 10-15 minutes. 1, 3
• If the repeat rapid test is negative, the patient can be reassured, though window period considerations apply (third-generation tests have a median window period of 22 days post-exposure). 6
• If the repeat rapid test is positive, confirmatory testing with supplemental assays (Western blot or fourth-generation antigen/antibody test) is mandatory before making a diagnosis. 1, 7

Critical Counseling Points

Reassure the patient that the line appearing after water washing does not indicate HIV infection. 1

• The original negative result during the valid reading window is what matters clinically. 1
• Rapid tests must be interpreted exactly as instructed—any deviation from protocol invalidates the result. 3
• The sensitivity and specificity of properly performed rapid tests exceed 99.5%, but this only applies when tests are performed and read correctly. 1, 8

Common Pitfalls to Avoid

• Never interpret rapid test results outside the specified reading window, as late-appearing lines are common artifacts and do not represent true positives. 1, 3
• Never manipulate test cassettes after reading (washing, adding additional sample, etc.) as this will create invalid results. 2, 3
• Do not attempt to "enhance" faint lines or unclear results by adding water or other substances—simply repeat the test with a new kit. 2
• Remember that even valid positive rapid tests require confirmatory testing before diagnosis, as the positive predictive value depends on prevalence in the tested population. 1