Carryover studies are not required for compliance. Some accreditation agencies and many manufacturers do require carryover studies. In addition, carryover studies are reflective of good laboratory practice. The purpose of the study is to determine if samples with high target values (HTV's) of the test parameter(s) impact subsequent samples that have low target values (LTV's) of the test parameter(s). The goal of the studies is for results to reflect widely separated concentration differences between the HTV and the LTV. Acceptable carryover limits are published by the manufacturer. Example: less than 3% carryover for white blood cells.
Fresh blood samples may be selected from the laboratory population. Additional samples may be prepared by either concentration or dilution of a single sample to obtain a sample proximal to the HTV or LTV. Other appropriate commercial samples, as defined by the manufacturer may be used. Commercial controls and air aspirations may not be substituted for fresh blood samples. Selected samples have high target values (HTV's) of the test parameter(s) and low target values (LTV's) of the test parameter(s) being evaluated. A minimum of three samples must be evaluated for a minimum of three repetitions to obtain a total of 9 values.
These high and low target samples are analyzed with multiple repetitions as directed by the manufacturer. A minimum of three repetitions of a sample with HTV's are tested. Immediately following the HTV testing samples with LTV's of the test parameter are tested. This process is repeated for each of the two remaining samples. This achieves the total of three samples with three repetitions required for the carryover samples.
The percent carryover is calculated as recommended by the manufacturer. The results of carryover are compared to the manufacturer's published acceptable limits. For example, if the manufacturer's acceptable carryover limit for hemoglobin is < 3.0 % and the calculated carryover for the instrument is 1.5%, then the calculated value is acceptable.
If the results are outside of the acceptable limits, corrective action must be taken to correct the discrepancy.
