The viral load in all sixteen samples ranged from 630 to 12 000 copies/ml by NASBA test, from 2930 to 102 000 copies/ml by RT-PCR test, and from 1816 to 23 561 by bDNA test, respectively. Viral load values were higher by RT-PCR than that by bDNA or NASBA (RT-PCR > bDNA > NASBA). The difference between RT-PCR and bDNA was 0.43 (CI 95%, 0.083–0.776); 0.40 (CI 95%, 0.214–0.578) for bDNA and NASBA; and 0.83 (CI 95%, 0.520–1.131) for RT-PCR and NASBA.
The mean bias in viral load between RT-PCR and bDNA assays is 0.429 log. The mean difference between bDNA and NASBA assays is 0.401 log. The mean difference between RT-PCR and NASBA assays is 0.830 log.
Viral load measurement has become a routine procedure for therapeutic management of HIV-1infected patients. Unfortunately, recombination has increased quickly and become more popular. Given the importance of accurate viral load measurements for optimal management of patients, the viral load assay must also reliably quantify genetically divergent strains of virus. In fact, several recent studies have raised concerns regarding the reliability of the results that can be obtained from the different commercially-available viral load assays – each of which is based on a different principle – when measuring highly polymorphic targets [1,7,11]. Thus, it is important that the performance of different viral load assays is critically evaluated for different HIV subtypes.
The ability to accurately quantitate all HIV-1 subtypes by different assays may vary significantly. RT-PCR has been reported to give higher values than bDNA for HIV-1 subtypes B, C, and D, while giving lower values for subtypes A, E, and F. The values across the subtypes for bDNA and RT-PCR differed by as much as 3– 450-fold. RT-PCR does not detect subtypes A, E, and F with the same efficiency as it detects subtypes B, C, and D (the fitted regression line was y = 0.733x + 1.627)  and this may be related to the design of the primers and probes in the RT-PCR assay. Sequence variation among different HIV-1 subtypes may cause mis-matches to the primer or probes, resulting in reduced hybridization and therefore under-quantitation of some subtypes .
The genetic variation between the sixteen samples and China subtype CRF_BC is less than 6.0%. Also this result indicated that the ratio of CRF_BC subtype in injected drug users (IDU) is very high in China mainland, especially in the west of China.
The three assays (RT-PCR, bDNA, NASBA) showed good correlation. Thus we surmised that the three assays have good correlation while quantifying RNA viral load of CRF_BC subtype of China with RT-PCR, bDNA, NASBA.
The authors would like to thank Dr. Sheena Sullivan for critical review of manuscript.
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