Preanalytical errors are those that are performed prior to the analytic phase of testing. Monitoring these errors is a vital part of a laboratory's quality assurance program. These are some of the most difficult errors to monitor as some of the processes in the preanalytical phase are out of the control of the laboratory. Examples of preanalytical errors that may affect hematology testing include: patient identification errors, specimen mix-ups, or compromise of specimen integrity.
One way to monitor preanalytical errors is through delta check alerts. A delta check compares current results for specific analytes/parameters to previous (consecutive) results. If the differences exceed predetermined limits for the predetermined length of time, the instrument operator is alerted by the instrument/information system so that the the discrepancy can be investigated. Analytes/parameters chosen for delta checks are those that show little short-term biologic variability. Deviations in these analytes/parameters are unlikely to be caused by changes in patient status. In the hematology laboratory, MCV and MCHC are parameters that are often used as delta checks. These parameters are stable in the blood for a short period of time (at least 24 hours). Events, such as acute hemorrhage, that may affect other blood count parameters do not affect MCV and MCHC within a 24-hour period. Rare exceptions, such as red blood cell transfusion and intravascular hemolysis exist, but the majority of acute conditions will not change these red blood cell indices over a short period of time.
Each laboratory must establish appropriate delta check triggering limits for the normal patient population. This can be done by analysis of pairs of data points from a representative population. After evaluation of the population, appropriate limits, 95 or 99%, can then be selected. Published data on diurnal variability may also be used. Once initially established, these limits may be modified.
The use of MCV to detect specimen mix-ups or incorrect patient identification is easily executed with the assistance of instrument flags. For example, if a CBC is ordered on a patient and it produces an MCV result of 90 fL and then another CBC is ordered on that patient 12 hours later and the MCV is 79 fL, the result should be investigated prior to reporting the second CBC. There is a possibility that one of the samples was mislabeled and that the two tubes were not from the same patient.
MCHC is likely to be affected by hemolysis with hemoglobinemia while the MCV will remain stable. However, the effect of hemolysis on the MCHC may be incorporated as a quality indicator for detection of sample hemolysis that occurs during collection or handling. Delta checks incorporating MCHC may have the added benefit of helping to detect analyzer malfunction. The MCHC is calculated from hemoglobin, MCV, and RBC counts. Any malfunction in any of the elements incorporated into this calculation will affect the MCHC.