Delta Check

Delta check is a process to detect discrepancies in patient test results prior to reporting by comparing current patient values to previous ones. Delta check limits define the allowable difference between consecutive results for a specific analyte on the same patient within a certain time interval. Delta check limits should be set so that true changes in patient test results are not flagged but improbable changes are flagged as delta check failures. Delta check limits should be based upon the total expected variation, which include biological and analytical variation (see Appendix A for biological variation).

Delta check limits can be expressed as the absolute or the percent difference between two consecutive results. Absolute limits are calculated as the difference between the larger and smaller result. Percent delta limits are calculated as the difference between the larger result and the smaller result divided by the smaller result. Time interval is the specimen collection time difference between the current and previous results. Time interval is flexible. Most hospital laboratories choose 24 or 48 hours.

Delta checks are recommended for inpatient testing. Generally, you want to select chemistry analytes that have the lowest biological variation. Below is an example delta limits for some common chemistry analytes.

 

 

Analyte Delta Limit Time Frame
Sodium 8 meq/L 30 hours
Chloride 8 meq/L 30 hours
Potassium 20% 30 hours
Carbon Dioxide 10 meq/L 30 hours
Creatinine 1.0 mg/dL 30 hours
BUN 20 mg/dL 30 hours
Calcium 2.0 mg/dL 30 hours
Magnesium 1.0 mg/dL 30 hours
Phosphorus 2.0 mg/dL 30 hours
Albumin 1.5 mg/dL 30 hours
Total protein 2.0 mg/dL 30 hours

 

To detect specimen mix-ups in hematology, delta checks should be applied to parameters that show the least short-term biological variation. MCV and MCHC are extremely stable in a patient over a short interval, such as 24 hours. The diurnal biological coefficient of variation in MCV is only 0.5%. Even in medical situations where other hematological parameters are changing rapidly, such as hemorrhage, MCV and MCHC do not change significantly since the reticulocyte response does not begin for two to three days. MCHC has the added benefit of detecting instrument malfunction because it is calculated from hemoglobin, MCV and RBC count.  These three parameters are directly measured. Suggested delta check limits for MCV and MCHC are +/-5 fl and +/-5.0 g/dL, respectively.

Delta checks are not recommended for other hematology parameters including hemoglobin, hematocrit, RBC count, WBC count or platelet count. Acute changes in these parameters are common in hospital patients and the false positive rate is unacceptably high. Some laboratories include a delta check for platelet count to catch specimens with platelet clumping.

Delta failure is an indicator of possible preanalytical error such as specimen mix-ups, patient mix-ups, or IV contamination of the specimen. When a delta failure occurs, the medical laboratory scientist should carefully review all results prior to releasing. Numerous delta failures on the same analyte may indicate an analytical problem with the instrument and should be investigated immediately.

Clinical correlation plays an important role in interpreting delta failures. For example, creatinine results in patients with renal failure rise and fall depending on their dialysis schedule. Patients receiving contrast media for imaging often have elevated creatinine values postprocedure that return to normal within a few days.

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