Errors of Classification With Potassium Blood Testing: The Variability and Repeatability of Critical Clinical Tests

Mayo Clin Proc. 2018 May;93(5):566-572. doi: 10.1016/j.mayocp.2018.03.013.


Objective: To understand the performance of a currently used clinical blood test with regard to the frequency and size of variation of the results.

Patients and methods: From November 29, 2012, through November 29, 2013, patients were recruited at 65 sites as part of a previously reported clinical trial ( Identifier: NCT01737697). Eligible outpatients who had been fasting for at least 8 hours underwent venous phlebotomy at baseline, 30 minutes, and 60 minutes to measure plasma potassium levels in whole blood using a point-of-care device (i-STAT, Abbott Laboratories). We analyzed the results to assess their variability and frequency of pseudohyperkalemia and pseudonormokalemia.

Results: A total of 1170 patients were included in this study. Absolute differences between pairs of measurements from different time points ranged from 0 to 2.5 mmol/L, with a mean difference of 0.26 mmol/L. The mean percentage differences were approximately 5% with an SD of 5%. Approximately 12% of differences between repeated fasting potassium blood test results were above 0.5 mmol/L (33% of the normal range), and 20% of patients (234) had at least one difference greater than 0.5 mmol/L. In 44.0% of the patients with a hyperkalemic average value (true hyperkalemia) (302 of 686), at least one blood test result was in the normal range (pseudonormokalemia), and in 30.2% of the patients with a normal average value (146 of 484), at least one blood test result was elevated (pseudohyperkalemia).

Conclusion: Expected variability and errors exist with potassium blood tests, even when conditions are optimized. Pseudohyperkalemia and pseudonormokalemia are common, indicating a need for thoughtful clinical interpretation of unexpected test results.

MeSH terms

  • Blood Specimen Collection
  • Critical Illness*
  • Diagnostic Errors / statistics & numerical data*
  • Female
  • Humans
  • Hyperkalemia / blood
  • Hyperkalemia / diagnosis*
  • Male
  • Potassium / blood*
  • Reference Values
  • Risk Factors


  • Potassium

Associated data