Pre-examination (formerly preanalytical) errors are those performed before the examination (formerly analytical) phase of testing. Monitoring these errors is vital to a laboratory's quality assurance program. These are some of the most difficult errors to monitor, as some of the processes in the pre-examination phase are out of the laboratory's control. Pre-examination mistakes that may affect hematology testing include patient identification errors, specimen mix-ups, or compromised integrity.
One way to monitor pre-examination 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 discrepancy can be investigated. Analytes/parameters chosen for delta checks are those that show little short-term biological 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 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 most acute conditions will not change these RBC indices over a short period.