BACKGROUND: Despite the high specificity of fourth-generation enzyme 
immunoassays (4th-gen-EIA) for screening during HIV diagnosis, their positive 
predictive value is low in populations with low HIV prevalence. Thus, screening 
should be optimized to reduce false positive results.
OBJECTIVES: The influence of sample cutoff (S/CO) values by a 4th-gen-EIA with 
the false positive rate during the routine HIV diagnosis in a low HIV prevalence 
population was evaluated.
STUDY DESIGN: A total of 30,201 sera were tested for HIV diagnosis using Abbott 
Architect® HIV-Ag/Ab-Combo 4th-gen-EIA at a hospital in Spain during 17 months. 
Architect S/CO values were recorded, comparing the HIV-1 positive results 
following Architect interpretation (S/CO≥1) with the final HIV-1 diagnosis by 
confirmatory tests (line immunoassay, LIA and/or nucleic acid test, NAT). ROC 
curve was also performed.
RESULTS: Among the 30,201 HIV performed tests, 256 (0.85%) were positive 
according to Architect interpretation (S/CO≥1) but only 229 (0.76%) were 
definitively HIV-1 positive after LIA and/or NAT. Thus, 27 (10.5%) of 256 
samples with S/CO≥1 by Architect were false positive diagnose. The false 
positive rate decreased when the S/CO ratio increased. All 19 samples with S/CO 
≤10 were false positives and all 220 with S/CO>50 true HIV-positives. The 
optimal S/CO cutoff value provided by ROC curves was 32.7. No false negative 
results were found.
CONCLUSIONS: We show that very low S/CO values during HIV-1 screening using 
Architect can result HIV negative after confirmation by LIA and NAT. The false 
positive rate is reduced when S/CO increases.