Example of use of clock time-dependent targets for patient-based quality control.

Background: Running means for total calcium (Ca) results at our laboratory exhibit a stable time-of-day (TOD) periodic pattern. We examined use of TOD-dependent targets for running means in patient-based quality control (PBQC) for Ca.

Methods: Primary data were Ca results over a 3 month interval, restricted to weekday data within the Ca reference interval (8.5-10.3 mg/dL; 2.12-2.57 mmol/L). Running means were evaluated as sliding averages of 20 samples (20-mers).

Results: Data comprised 39,629 consecutive Ca measurements (75.3% inpatient (IP)) for which Ca was 9.29±0.47 mg/dL. The all data average for 20-mers was 9.29 ± 0.18 mg/dL. When parsed in 1 h TOD intervals, however, averages among 20-mers ranged from 9.1 to 9.5 mg/dL, with blocs of contiguous results above (0800-2300 h; 53.3% of results; IP = 75.3%) and below (2300-0800 h; 46.7% of results; IP = 99.9%) the all-data mean. There was thus an inherent TOD-dependent pattern of deviation of means from target when using a fixed PBQC target. Using Fourier series analysis as an example approach, characterization of the pattern to produce TOD-dependent PBQC targets eliminated this inherent inaccuracy.

Conclusions: In circumstances of periodic variation in running means, simple characterization of that variation can reduce the probability of both false positive and false negative flags in PBQC.