PI Perspective #17

December 1995     View PDF     En español

Comparing RNA PCR Tests

Tests which measure viral burden (HIV RNA levels in blood) are a sensitive new tool for examining the effect of HIV therapies. As they become widely available, it becomes important for people to know which tests they are getting and how test results compare from one version to another. The most commonly used tests are: Q-PCR: (quantitative polymerase chain reaction, or the Roche PCR); bDNA (branched-chain DNA); and NASBA: (nucleic acid sequence-based amplification). NASBA is commonly used in Europe and is beginning to show up in the U.S. Different laboratories may use different methods of detecting viral burden including their own home brew test.

Studies suggest the tests give similar results, although NASBA may produce slightly higher HIV RNA copy numbers. The bDNA test is generally more consistently reproducible than Q-PCR or NASBA with very little difference between these two. The ACTG virology laboratories found Q-PCR and bDNA comparable in direct comparisons.

The tests have different ranges in which they measure HIV RNA levels. The bDNA test is most sensitive in the 10,000 to 1,000,000 HIV RNA copy range, NASBA is most sensitive between 1,000 to 10,000,000 copies, and Q-PCR between 200 to 1,000,000. A test may be more or less appropriate depending on the stage of HIV disease. Those with high CD4+ cell counts may only have detectable virus load on the Q-PCR test. Also, different amounts of plasma are needed for the tests—bDNA uses 2 ml of plasma (several tablespoons) while NASBA and Q-PCR use 100 and 200 microliters respectively (much less). This may be important when these tests are also used to detect other organisms which cause disease in people with HIV such as cytomegalovirus (CMV) and mycobacterium avium complex (MAC) and therefore requiring large amounts of blood for the tests. Talk to your physician to find out what test is being used, how sensitive it is, and how large a change is needed to be considered significant.