Is the Insti (HIV) test reliable for detecting HIV (Human Immunodeficiency Virus) infection?

Is the INSTI HIV Test Reliable?

The INSTI HIV-1/HIV-2 antibody test is reliable for detecting chronic HIV infection with sensitivity and specificity comparable to other rapid tests (>99% for established infection), but it has significant limitations in detecting acute/early HIV infection, missing 20-30% of early seroconversion cases detected by laboratory-based assays. 1, 2

Performance in Established HIV Infection

• The INSTI test demonstrates excellent performance for chronic HIV infection, with sensitivity and specificity both greater than 99.5% when compared to standard laboratory-based assays 2
• It provides results in under 5 minutes and requires no specialized equipment, making it practical for point-of-care settings 2
• The test reliably detects antibodies to HIV-1 (groups M and O) and HIV-2 in established infections 2

Critical Limitations in Early/Acute HIV Infection

The INSTI test performs poorly during the window period and early seroconversion, which is when patients are most infectious and transmission risk is highest. This has direct implications for morbidity and mortality through missed diagnoses and continued transmission.

• In early seroconversion cases (p24 antigen positive but antibody negative/indeterminate), INSTI detected only 69.4% of infections that were positive by third-generation laboratory EIA 1
• For the earliest infections (Western blot negative with positive RNA and p24 antigen), INSTI sensitivity drops dramatically to only 7% 3
• The test missed 20-30% of early infections detected by standard laboratory-based third-generation antibody tests 1
• INSTI detects seroconversion later than standard laboratory-based assays, creating a diagnostic gap during the acute infection period 2

Comparison to Fourth-Generation Tests

Fourth-generation antigen/antibody combination assays are superior for early detection and should be the preferred initial screening test. 4

• Fourth-generation tests simultaneously detect both HIV antibodies and p24 antigen, reducing the window period by approximately 4 days compared to third-generation antibody-only tests 4
• These combination assays detect 89% of all recent infections and 80% of acute infections (RNA positive, antibody negative) 5
• The CDC recommends fourth-generation antigen/antibody combination assays as the initial screening test for HIV diagnosis, with sensitivity and specificity greater than 99.5% 4

Clinical Recommendations

Use INSTI only when fourth-generation laboratory testing is unavailable or when immediate results are required, and always confirm positive results with laboratory-based testing. 4

• For suspected acute HIV infection (recent high-risk exposure, symptoms of acute retroviral syndrome), do not rely on INSTI alone—obtain both a fourth-generation antigen/antibody test and HIV RNA nucleic acid amplification testing (NAAT) 4
• Any positive INSTI result requires confirmation with HIV-1/HIV-2 antibody differentiation immunoassay 4
• If INSTI is negative but clinical suspicion for acute infection remains high, proceed immediately to NAAT testing, as antibody-based tests (including INSTI) will miss the earliest infections 4

Common Pitfalls to Avoid

• Never use INSTI as the sole test for post-exposure prophylaxis (PEP) evaluation—the window period limitations could miss acute infections when intervention is most critical 4
• Do not assume a negative INSTI result excludes HIV in patients with recent exposure (within 4-6 weeks)—the test may be negative during the antibody-negative window period 1, 3
• Avoid using INSTI for screening in high-incidence populations where acute infections are more prevalent—the lower sensitivity for early infection increases the risk of missed diagnoses and ongoing transmission 3
• In infants under 18 months born to HIV-positive mothers, antibody tests including INSTI are unreliable due to maternal antibody transfer—definitive diagnosis requires nucleic acid testing or antigen detection 4