Occurrence of false positivity in a fourth generation (Ag/Ab) HIV screening assay: horns of a dilemma

Partha Roy, Ruchi Kapoor, Priya Rawat, Monika Aggarwal, Ravi Gaur, Mrunalini Anand, Pallav Saharia


Background: Universal screening for HIV with Ag/Ab combo assays has reduced the window period significantly due to its high sensitivity. However, occurrence of false positives is common and in a country like India, it may lead to social distress and loss of livelihood. We wanted to ascertain the occurrence of false positives in our laboratory.

Methods: There were 21817 samples analysed retrospectively from Jan 2015 to Jul 2017 (31 months). Architect HIV Ag/Ab Combo (Abbott Laboratories, Abbott Park, IL) was used as the first test. Repeatedly reactive samples with a signal to cutoff (S/CO) ratio greater than or equal to 1.00 was considered reactive. Strategy III (3 test algorithm) of NACO guidelines was followed uniformly.

Results: In this study, 147 samples tested reactive (0.67%) and 40 samples tested false positive (repeatedly reactive; 0.18%). 6 samples were indeterminate (0.027%). Overall, the sensitivity of the Architect HIV Ag/Ab Combo was 100%, the specificity varied between 99.74% and 99.91%. The PPV from June 2016 to July 2017 was 68.63% (32 false positives). The S/CO values of 72.8% (117/147) reactive samples ranged between 201- 800, whereas 72% (29/40) of false positive samples, the S/CO values ranged between 1.0-2.0. The specificity of the test improved to 99.98% when S/CO value was adjusted at 2 and 100% when adjusted at 5. Similarly, the PPV too improved to 93.04% and 98.66% at S/CO values of 2 and 5 respectively.

Conclusions: Further studies are needed to ascertain the optimal or ‘grey-zone’ S/CO values for India to minimise the false positive results and avoid further supplemental tests routinely.


Ag/Ab combo assay, False positive, HIV, Test sensitivity, S/CO

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Weber B, Fall EH, Berger A, Doerr HW. Reduction of diagnostic window by new fourth-generation human immunodeficiency virus screening assays. J Clin Microbiol. 1998;36:2235-9.

Weber B, Gurtler L, Thorstensson R, Michi U, Muhlbacher A, Burgisser P, et al. Multicenter evaluation of a new automated fourth generation human immunodeficiency virus screening assay with a sensitive antigen detection module and high specificity. J Clin Microbiol. 2002;40:1938-46.

Guinn D. HIV screening and false-positive results. JAMA. 2007;297:947.

Branson B, Handsfield MHH, Lampe MA, Janssen RS, Taylor AW, Lyss SB, et al. The Centers for Disease Control and Prevention. Revised recommendations for HIV testing of adults, adolescents, and pregnant women in health-care settings. MMWR Recommend Rep. 2006;55:1-17.

National AIDS Control Organisaiton. National Guidelines for HIV Testing. Ministry of Health and Family Welfare, Government of India. July 2015.

Shima-Sano T, Yamada R, Sekita K, Hankins R W, Hori H, Seto H, et al. A human immunodeficiency virus screening algorithm to address the high rate of false-positive results in pregnant women in Japan. PLoS One. 2010;5:e9382.

Chavez P, Wesolowski L, Patel P, Delaney K, Owen SM. Evaluation of the performance of the Abbott Architect HIV Ag/Ab Combo assay. J Clin Virol. 2011;52(Suppl):S51-55.

Kwon JA, Yoon SY, Lee CK, Lim CS, Lee KN, Sung HJ, et al, Performance evaluation of three automated human immunodeficiency virus antigen-antibody combination immunoassays. J Virol Methods. 2006;133:20-6.

Song EY, Hur M, Roh EY, Park MH, Moon H-W, Yun YM. Performances of four fourth-generation human immunodeficiency virus-1 screening assays. J Med Virol. 2012;84:1884-8.

US Food and Drug Administration. Abbott ARCHITECT HIV Ag/Ab Combo. 2014.

US Food and Drug Administration. Abbott ARCHITECT HIV Ag/Ab Combo Insert. 2009. Available at: BiologicsBloodVaccines/BloodBloodProducts/ApprovedProducts/LicensedProductsBLAs/BloodDonorScreening/InfectiousDisease/UCM216312.pdf.

US Food and Drug Administration. Abbott ARCHITECT HIV Ag/Ab Combo PMA; Summary of Safety and Effectiveness Data (SSED).

Kim S, Lee JH, Choi JY, Kim JM, Kim H-S. False-positive rate of a “fourth-generation” HIV antigen/antibody combination assay in an area of low HIV prevalence. Clin Vaccine Immunol. 2010;17:1642-4.

Jensen TO, Robertson P, Whybin R, Chambers I, Lahra M, Rawlinson W, Post JJ. A signal-to-cutoff ratio in the Abbott Architect HIV Ag/Ab Combo assay that predicts subsequent confirmation of HIV-1 infection in a low prevalence setting. J Clin Microbiol. 2015;53:1709-11.

Munshi SU, Anwar A, Tabassum S. False positive human immunodeficiency virus antibody test in chronic hepatitis B patient. Indian J of Medical Microbiol. 2014;32:344-5.

Liu P, Jackson P, Shaw N, Heysell S. Spectrum of false positivity for the fourth-generation human immunodeficiency virus diagnostic tests. AIDS Research and Therapy. 2016;13:1

Klarkowski D, O'Brien DP, Shanks L, Singh KP. Causes of false-positive HIV rapid diagnostic test results. Expert Rev Anti Infect Ther. 2014;12(1):49-62

Gray RH, Makumbi F, Serwadda D, Lutalo T, Nalugoda F, Opendi P, et al. Limitations of rapid HIV-1 tests during screening for trials in Uganda: diagnostic test accuracy study. BMJ. 2007;335:188.