The use of serial outpatient complete blood count ( CBC ) results to derive biologic variation: a new tool to gauge the acceptability of hematology testing

Summary Introduction Most estimates of biologic variation (s b ) are based on periodically acquiring and storing specimens from reference subjects, followed by analysis within a tightly controlled analytic run. We demonstrate that reliable estimates of s b can be derived for virtually all constituents of the CBC from previously obtained paired patient results and summary QC data. Methods A laboratory data repository provided all of the outpatient CBC results measured over 20.5 months at a large Canadian referral laboratory. These CBC measurements were taken on one of four Beckman Coulter LH analyzers. A total of 1852 different patients had CBC s repeated at least twice within 84 h. We tabulated the pairs of intrapatient constituents that were separated by 0–6, 6–12, 12–18,… 72–78, and 78–84 h. The standard deviations of duplicates ( SDD ) of the paired data were then regressed against time. The y‐intercept represents the sum of s b and short‐term analytic variation (s a ): y 0 =(s 2 a +s 2 b ) 1/2 . The short‐term imprecision was determined from normal range Coulter quality control specimens. Results Patient s b for hematocrit, MCH , absolute monocytes, and absolute neutrophils are extremely close to those determined by biologic variation experiments using healthy volunteers. Most of the other estimates of s b tended to be slightly lower than literature estimates. Conclusions We describe a novel approach to deriving s b . The ratio of the s b to s a (a measure of sigma) indicates that the Beckman Coulter LH is extremely suitable for CBC monitoring of outpatients as well as for inpatients, whose s b is generally higher.