Potential clinical implications of interlaboratory variability in CD4+ T-lymphocyte counts of patients infected with...

The CD4+ T-lymphocyte count is an important factor in the management of patients infected with human immunodeficiency virus. Previous studies have found significant variability among the counts determined by different laboratories. We conducted a study of lymphocyte phenotyping in four laboratories to assess this variability and its possible clinical implications. One laboratory was situated at the study site; the other three were selected randomly from a total of 11 commercial and hospital laboratories available locally. Blood specimens were obtained from 24 patients and were sent to the four laboratories for a complete blood count and a lymphocyte subset analysis. Using the Kruskall-Wallis test, we found that the laboratories' ranks of four individual components of the CD4 cell count differed significantly: total white blood cell count (P < .0001), lymphocyte percentage (P = .003), lymphocyte count (P = .002), and CD4 percentage (P = .0004). Of the 24 patients in this survey, 14 (58.3%) had CD4-count results with enough variation to have led to conflicting treatment recommendations; three of the 24 patients fulfilled the revised Centers for Disease Control and Prevention case definition of AIDS on the basis of results from some but not all laboratories. In addition, the laboratories disagreed on whether CD4 cell counts of nine patients (37.5%) had increased or decreased since the previous determination. We conclude that when strict thresholds of CD4 cell counts are used as a basis for treatment recommendations or for diagnosis of AIDS, interlaboratory variability may be sufficient to alter the decisions made.