A delta check is a quality control tool that involves the comparison of laboratory test results with results obtained on previous samples from the same patient. Delta checks can be programmed into the laboratory’s computer system to detect an error. For example, in the hematology laboratory, a delta check for MCV would compare a patient's current MCV of 80 fL to their last value, obtained perhaps 12 hours ago, which was 95 fL. This significant change could indicate real pathology, or an issue with the testing or test sample. The lab's computer system would alert the instrument operator to this significant change.
Delta checks are particularly useful for detecting errors in specimen identification, specimen integrity, errors in manual data entry, or possible analytical errors. These are quality control issues that usually cannot be detected by testing quality control materials.
For most tests, it is unlikely that consecutive results obtained on one patient will vary significantly unless a substantial change has occurred with the patient’s medical status. The majority of delta check failures are caused by changes in patient status. However, if multiple delta checks fail on several tests performed on a single patient, there is a strong possibility that the patient or specimen was misidentified.
Delta checks are not useful for every analyte; they lend themselves best to stable analytes with little day-to-day variation that are measured frequently such as red blood cell (RBC) indices, electrolytes, or liver function tests.