The use of HIV RNA assays for clinical purposes requires specific considerations (43). Several different methods can be used for quantifying HIV RNA, each with different levels of sensitivity. Although the results of the assays are correlated, the absolute HIV RNA copy number obtained from a single specimen tested by two different assays can differ by twofold (0.3 log10) or more.
One HIV RNA assay method should be used consistently for monitoring each patient.
Biologic variation in HIV RNA levels within one person is well documented, and repeated measurement of HIV RNA levels in a clinically stable infected adult can vary by as much as threefold (0.5 log10) in either direction over the course of a day or on different days (5, 42, 47). This biologic variation may be greater in infected infants and young children.
In children with perinatally acquired HIV infection, RNA copy number slowly declines even without therapy during the first several years after birth, although it persists at higher levels than those observed in most infected adults (23, 37, 38). This decline is most rapid during the first 12–24 months after birth, with an average decline of approximately 0.6 log10 per year; a slower decline continues until approximately age four to five years (average decline of 0.3 log10 per year).
This inherent biologic variability must be considered when interpreting changes in RNA copy number in children.
Only changes greater than fivefold (0.7 log10) in infants aged <2 years and greater than threefold (0.5 log10) in children aged >2 years after repeated testing should be considered reflective of a biologically and clinically substantial change. To reduce the impact of assay variability in the clinical management of patients, two samples can be obtained at baseline and the average of the two values used for comparison with future tests.