Literature review > Issue_5 > Review on Cherpes et al. 

Plasma and serum specimens have long been used interchangeably in HIV and other enzyme immunoassays (EIA) and rapid tests, apparently producing equivalent results [1,2]. Serum is preferred over plasma as a specimen, however, in the occasional EIA and other test formats, especially when quantitative or semi-quantitative results are required [3,4]. This brief report by Cherpes et al., is therefore useful because it indicates that plasma is equivalent to serum when used in an HSV-2-specific EIA (Focus Technologies, Cypress, CA). A 98.9% concordance between sera and plasmas was reported. In 309 specimens tested as positive there were only nine (2.9%) discordant results, eight of which were positive with plasmas but not sera, and one that was positive with serum but not plasma. The true serological status of these nine specimens, however, was not confirmed. If Western blots or another confirmatory test had been performed, these results would perhaps have been more interesting. These data could have been used to suggest that plasmas were more sensitive than sera in this assay, or that they were false positives. This would be important if test results were returned to the patients but less important if patients were not informed and these data were used for surveillance. As this report now stands, without these confirmatory data, it would be difficult to recommend using plasma over serum for diagnostic testing without a supplemental confirmatory test.

In general, this study is well designed. The sample size is more than adequate given the levels of concordance between the two sample types. Also, given that this is a quasi-discrepancy analysis, the authors do mention retesting the discordant specimens but finding no change in the original results. This resolves a potential source of bias in the trial. If two additional bits of data had been presented, they would have improved and further illuminated this analysis. First, the authors give an example of the concordance between high positive antibody titers (7.55 versus 7.37). It would have been helpful to graphically or numerically represent the agreement between the two sample types at different antibody titers, possibly by tertiles or quartiles, which would provide insight into where in the distribution of antibody titers the assay performance diverges most strongly. Second, a tabular presentation of the nine discordant samples along with antibody titer values for both sample types and possibly other demographic and infection status variables would have allowed the reader to hypothesize about the possible causes for the disparities between the two.

Since plasma and serum specimens do show a high degree of concordance in this study, collection of plasmas may result in a considerable cost savings compared to sera when a panel is collected prospectively, since plasmas may be more easily handled and processed from venous blood. This would also allow retrospective testing of specimen panels collected as plasmas without having to subject them to defibrination to obtain sera [5]. We await a follow-on report from these or other investigators validating the use of blood spot specimens with this test as equivalent to serum and plasma to further expand the utility of this HSV-2 test.

References:

1. Ketema F, et al. Assessment of the performance of a rapid, lateral flow assay for the detection of antibodies to HIV. JAIDS. 2001, 1; 27:63-70

2. Luyasu V, at al. Multicenter evaluation of a new commercial assay for detection of immunoglobulin M antibodies to Toxoplasma gondii. Eur J Clin Microbiol Infect Dis. 1995, 14:787-93.

3. Larsen SA, Syphilis. Clin Lab Med. 1989 9:545-557.

4. Hix J, et al. Development of a rapid enzyme immunoassay for the detection of retinol-binding protein. Am J Clin Nutr. 2004; 79:93-8

5. Castro AR, et al. Defibrination of blood plasma for use in serological tests for syphilis. Clin Diagn Lab Immunol. 2002; 9:1376-8..

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