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Editor,—Owing to a higher risk of HIV transmission during seroconversion and a rise in the transmission of resistant HIV,1 it is becoming increasingly important to diagnose primary HIV infection (PHI).2 However, the diagnosis can be difficult to make as there is a wide differential diagnosis and conventional HIV antibody tests do not become positive until an average of 8 weeks after infection. HIV p24 antigen tests become positive at an earlier stage and are often used as part of a combined screening test for suspected PHI. These combined tests detect p24 antigenaemia, anti-HIV-1, and anti-HIV-2 antibodies and can assist in earlier diagnosis of HIV. We would like to present a case of false positive p24 antigen using one such test.
A 26 year old man attended the same day HIV testing clinic requesting an HIV test. He complained of symptoms, which had been present for 3 weeks and were consistent with PHI: headaches, fatigue, night sweats and enlarged neck glands, and later a rash on his body, spreading to his arms and legs. His last sexual contact was a casual male partner 2 months previously, with whom he had unprotected oral sex. His only previous two partners were female. On examination he had a widespread maculopapular rash, generalised lymphadenopathy, and oral ulceration.
An STI screen was performed, including an HIV test (Biomérieux Vidas HIV Duo) and syphilis and hepatitis A/B serology. The HIV test was HIV p24 antigen positive, and anti-HIV-1/HIV-2 antibody negative. The patient was informed of the result and given a provisional diagnosis of PHI, although he was advised that further tests were required, the results of which would be available the following day. He was subsequently found to be HIV-1 and HIV-2 antibody negative by two methods (Abbott Axsym Microparticle Enzyme immunoassay or MEIA and Abbott Determine) and HIV RNA negative (Roche Amplicor). Furthermore, a syphilis EIA screening test (Abbott Murex ICE), was strongly positive. Hepatitis serology was negative.
Reference laboratory testing of the sample confirmed it to be HIV-1 and HIV-2 antibody and HIV p24 antigen negative. The syphilis result was confirmed at the reference laboratory as VDRL positive 1 in 32, Treponema pallidum particle agglutination test (TPPA) positive, titre > 1280, syphilis IgG ELISA positive, IgM ELISA positive. His symptoms rapidly resolved on treatment with intramuscular bicillin.
There have been very few reports of false positive p24 antigen tests and these have been mostly in transplant recipients using p24 antigen-only kits.4 Primary HIV infection and secondary syphilis can present with similar symptoms. Given that there are currently several ongoing outbreaks of syphilis in the United Kingdom,5 it is very important to distinguish between these two diagnoses.
In this case the HIV screening test was positive, although subsequent testing of this sample by other methods showed it to be negative for p24 antigen and HIV-1 and HIV-2 antibody. Although this false positive result may have been the result of the secondary syphilis infection, it should be noted that the Medical Devices Agency evaluation report on the Biomérieux Vidas HIV Duo kit found it to have a high specificity (99.73%). Of 372 HIV-1 negative sera that were tested one was found to be repeatedly reactive.6
In our enthusiasm to diagnose PHI, the clinical similarity to secondary syphilis must be remembered and caution taken in interpreting preliminary test results.
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