I want to inquire about verification of target mean/ranges for hematology analytes. We run a control material 20 times and calculate statistics such as mean, standard deviation, and coefficient of variation.

Editor: Frederick L. Kiechle, MD, PhD

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Q. I want to inquire about verification of target mean/ranges for hematology analytes. We run a control material 20 times and calculate statistics such as mean, standard deviation, and coefficient of variation. We also calculate total analytical error based on a formula (TAE = bias + 2 SD) and compare the TAE with the allowable total error recommended by CLSI and other sources. For example, if TAE for platelets (based on reading control material 20 times) is less than 25 percent (a CLSI recommended value), we accept the target range; otherwise, we reject it. However, since low concentrations of analytes are prone to a higher degree of variation, the aforementioned target range verification process frequently fails.

Is it necessary to accept or reject established target values based on total analytical error? Or is there an alternative way to do that?

A.It is not necessary to calculate total analytical error when establishing or verifying control ranges when implementing a new lot of control material. Repetitive analysis of the new lot to determine mean, standard deviation, and coefficient of variation is sufficient when establishing a new range for an unassayed control material or verifying a manufacturer’s range for an assayed control material. Acceptability limits for verifying a new lot of control are established by the laboratory director based on the requirements for patient care and assay performance in that specific laboratory and are included in the lab’s written procedure. Best practice is to use both an absolute value and a relative percentage when setting acceptability limits. The absolute limit will apply at low analyte concentrations, and the percentage limit will apply at high analyte concentrations.

Your lab is running into trouble at low platelet concentrations because it is using only a relative percentage limit (25 percent) and lacks an absolute limit for platelets. The lab could use an acceptability limit of target ± 25 percent or ± 10 × 10³/µL (10 × 10⁹/L), whichever is greater. In other words, the limit would be ± 25 percent for platelet counts of 40 or greater and ± 10 × 10³/µL (10 × 10⁹/L) for platelet counts of 40 or less.