title: How useful are delta checks in the 21 century? A stochastic-dynamic model of specimen mix-up and detection.
abstract: BACKGROUND
  Delta checks use two specimen test results taken in succession in order to detect test result changes greater than expected physiological variation. One of the most common and serious errors detected by delta checks is specimen mix-up errors. The positive and negative predictive values of delta checks for detecting specimen mix-up errors, however, are largely unknown.
  
  
  METHODS
  We addressed this question by first constructing a stochastic dynamic model using repeat test values for five analytes from approximately 8000 inpatients in Calgary, Alberta, Canada. The analytes examined were sodium, potassium, chloride, bicarbonate, and creatinine. The model simulated specimen mix-up errors by randomly switching a set number of pairs of second test results. Sensitivities and specificities were then calculated for each analyte for six combinations of delta check equations and cut-off values from the published literature.
  
  
  RESULTS
  Delta check specificities obtained from this model ranged from 50% to 99%; however the sensitivities were generally below 20% with the exception of creatinine for which the best performing delta check had a sensitivity of 82.8%. Within a plausible incidence range of specimen mix-ups the positive predictive values of even the best performing delta check equation and analyte became negligible.
  
  
  CONCLUSIONS
  This finding casts doubt on the ongoing clinical utility of delta checks in the setting of low rates of specimen mix-ups.
authors.affiliation: Bachelor of Health Sciences Program, Faculty of Medicine, Room G503, O'Brien Centre for the BHSc, 3330 Hospital Drive N.W, Calgary, Alberta T2N 4N1, Canada.
authors.forename: Katie
authors.initials: K
authors.lastname: Ovens
authors.forename: Christopher
authors.initials: C
authors.lastname: Naugler