HIV false positive screening serology due to sample contamination reduced by a dedicated sample and platform in a high prevalence environment.

Automated testing of HIV serology on clinical chemistry analysers has become common. High sample throughput, high HIV prevalence and instrument design could all contribute to sample cross-contamination by microscopic droplet carry-over from seropositive samples to seronegative samples resulting in false positive low-reactive results. Following installation of an automated shared platform at our public health laboratory, we noted an increase in low reactive and false positive results. Subsequently, we investigated HIV serology screening test results for a period of 21 months. Of 485 initially low positive or equivocal samples 411 (85%) tested negative when retested using an independently collected sample. As creatinine is commonly requested with HIV screening, we used it as a proxy for concomitant clinical chemistry testing, indicating that a sample had likely been tested on a shared high-throughput instrument. The contamination risk was stratified between samples passing the clinical chemistry module first versus samples bypassing it. The odds ratio for a false positive HIV serology result was 4.1 (95% CI: 1.69-9.97) when creatinine level was determined first, versus not, on the same sample, suggesting contamination on the chemistry analyser. We subsequently issued a notice to obtain dedicated samples for HIV serology and added a suffix to the specimen identifier which restricted testing to a dedicated instrument. Low positive and false positive rates were determined before and after these interventions. Based on measured rates in low positive samples we estimate that before the intervention, of 44 117 HIV screening serology samples, 753 (1.71%) were false positive, declining to 48 of 7 072 samples (0.68%) post-intervention (p<0.01). Our findings showed that automated high throughput shared diagnostic platforms are at risk of generating false-positive HIV test results, due to sample contamination and that measures are required to address this. Restricting HIV serology samples to a dedicated platform resolved this problem.

Contamination with HIV antibody may be responsible for false positive results in specimens tested on automated platforms running HIV 4th generation assays in a region of high HIV prevalence.

INTRODUCTION: In South Africa where the prevalence of HIV infection is very high, 4th generation HIV antibody/p24 antigen combo immunoassays are the tests of choice for laboratory based screening. Testing is usually performed in clinical pathology laboratories on automated analysers. To investigate the cause of false positive results on 4th generation HIV testing platforms in public sector laboratories, the performance of two automated platforms was compared in a clinical pathology setting, firstly on routine diagnostic specimens and secondly on known sero-negative samples. METHODS: Firstly, 1181 routine diagnostic specimens were sequentially tested on Siemens and Roche automated 4th generation platforms. HIV viral load, western blot and follow up testing were used to determine the true status of inconclusive specimens. Subsequently, known HIV seronegative samples from a single donor were repeatedly tested on both platforms and an analyser was tested for surface contamination with HIV positive serum to identify how suspected specimen contamination could be occurring. RESULTS: Serial testing of diagnostic specimens yielded 163 weakly positive or discordant results. Only 3 of 163 were conclusively shown to indicate true HIV infection. Specimen contamination with HIV antibody was suspected, based on the following evidence: the proportion of positive specimens increased on repeated passage through the analysers; viral loads were low or undetectable and western blots negative or indeterminate on problem specimens; screen negative, 2nd test positive specimens tested positive when reanalysed on the screening assay; follow up specimens (where available) were negative. Similarly, an increasing number of known negative specimens became (repeatedly) sero-positive on serial passage through one of the analysers. Internal and external analyser surfaces were contaminated with HIV serum, evidence that sample splashes occur during testing. CONCLUSIONS: Due to the extreme sensitivity of these assays, contamination with minute amounts of HIV antibody can cause a negative sample to test positive. Better contamination control measures are needed on analysers used in clinical pathology environments, especially in regions where HIV sero-prevalence is high.

Cytomegalovirus load in whole blood is more reliable for predicting and assessing CMV disease than pp65 antigenaemia.

CMV is a common cause of disease in immunocompromised patients. Because sampling of the diseased organ can be invasive, markers of systemic CMV reactivation such as pp65 and CMV viral load are commonly used to monitor patients at risk of CMV disease. In this retrospective analysis, the performance of these markers was compared in solid organ transplant recipients, patients with haematological malignancies and HIV infection. Both assays were sensitive markers of reactivation, however, the predictive value for disease of a positive result for both was low. Compared to viral load, the pp65 assay was a less sensitive marker of CMV reactivation. It was only positive when the viral load was greater than 3 log (10) copies/ml whole blood and was negative in 10 instances when the viral load was between 3 and 5 logs. In concordantly positive samples, the number of pp65 positive cells varied widely relative to the viral load and the number of positive cells counted could not be used to predict disease likelihood with any certainty. To conclude, CMV viral load provides a more consistent guide to determine likelihood of disease than pp65 count and is a more sensitive marker of CMV reactivation.