Clinical Utility of the Signal-to-Cutoff Ratio of Reactive HIV Antigen/Antibody Screening Tests in Guiding Emergency Physician Management

Background: The signal-to-cutoff (S/CO) ratio of the HIV antigen/antibody test may help immediately to differentiate true-positive results from false-positive results, which may be particularly useful in time-sensitive circumstances, such as when providing emergency department (ED) care.

Setting: Seven US EDs with HIV screening programs using HIV antigen/antibody assays.

Methods: This cross-sectional study of existing data correlated S/CO ratios with confirmed HIV status. Test characteristics at predetermined S/CO ratios and the S/CO ratio with the best performance by receiver operator characteristic (ROC) curve were calculated.

Results: Of 1035 patients with a reactive HIV antigen/antibody test, 232 (22.4%) were confirmed HIV-negative and 803 (77.6%) were confirmed HIV-positive. Of the 803 patients, 713 (88.8%) experienced chronic infections and 90 (11.2%) experienced acute infections. S/CO ratios were greater for HIV-positive (median 539.2) than for HIV-negative patients (median 1.93) (P < 0.001) and lower for acute infection (median 22.8) than for chronic infection (median 605.7) (P < 0.001). All patients with an S/CO ratio < 1.58 (n = 93) were HIV-negative (NPV 100%), and nearly all with an S/CO ≥ 20.7 (n = 760) (optimal level by ROC analysis) were HIV-positive (PPV 98.6%). Of patients with S/CO values between 1.58 and 20.7 (n = 182), 29.7% were HIV-positive.

Conclusions: The S/CO ratio may be used in real time to classify most ED patients as almost certain to be either HIV-positive or HIV-negative long before nucleic acid confirmatory testing is available. When combined with clinical judgment, this could guide preliminary result disclosure and management.