Correcting laboratory results for the effects of interferences: an approach incorporating uncertainty of measurement

Abstract

Background: Results of numerical pathology tests may be subject to interference and many laboratories identify such interferences and withhold results or issue warnings if clinically erroneous results may be issued. Some laboratories choose to correct for the effect of interferences, with the uncertainty of the correction noted as a limitation in this process. We investigate the effect of correcting for the effect of interferences on the ability to release results within defined error goals using the effect of in-vitro haemolysis on serum potassium measurement as an example. Methods: A model was developed to determine the uncertainty of a result corrected for the effect of an interferent with a linear relationship between concentration and effect. The model was used to assess the effect of correction on the results which could be released within specified accuracy criteria. Results: Using the effects of haemolysis on potassium results as an example, the maximum amount of haemolysis in a sample that would change the result by > 0.5 mmol/L, with a frequency of 5%, was increased from approximately 1100 mg/L (no correction) to 8000 mg/L (with correction). Conclusions: With modelling of the factors related to the uncertainties of results in the presence of interferences, it is possible to release results in the presence of significantly higher concentrations of interferences after correction than without correction. Correction of a result for a known bias and allowance for the uncertainty of the correction can be considered consistent with the guide to the expression of uncertainty in measurement (GUM).

Extra-analytical sources of uncertainty: which ones really matter?

“…Therefore, in some circumstances, some preanalytical events can effectively be considered sources of variations of test results (such as slight hemolysis, prolonged storage time or above the normal range of specimen temperature) and therefore included in the MU estimation. For example, an attempt for handling the uncertainty of test results for the effects of interferences on specimens, such as hemolysis, either by applying a correction factor or by increasing measurement uncertainty, have been proposed by Jones and Hawkins [26]. These authors evaluated the possibility of reporting results corrected for hemolysis interferences, under certain defined and limited circumstances, by including in MU the uncertainty of the correction factor.…”

Statistical correction of potassium in haemolysed samples

“…1 The authors suggest that correcting this form of interference would allow the release of more potassium results. They also emphasise that this approach is limited to the situation where the concentration of the interferent is measurable, with a linear relationship between concentration of interferent and its effect on the analyte of interest.…”

“…I read with interest the suggestion by Jones and Hawkins of applying a statistical model to quantitate and correct, using Microsoft Excel, the effect of interference of haemolysis (haemoglobin concentration) on potassium measurements. 1 The authors suggest that correcting this form of interference would allow the release of more potassium results. They also emphasise that this approach is limited to the situation where the concentration of the interferent is measurable, with a linear relationship between concentration of interferent and its effect on the analyte of interest.…”