To what extent can the actual level of ALP, ALT, or bilirubin differ from the measured value?

Uncertainty of laboratory measurement results

The result of any laboratory measurement of a given biological parameter is burdened with a certain degree of uncertainty. This is due to the principal impossibility of performing absolutely accurate measurements. The laboratory-determined result is therefore not a single number, but an interval within which the actual value is most likely to be found. Knowing the measurement uncertainty allows the laboratory and the recipient of the result to get an idea of its quality.¹

The uncertainty of the measurement result consists of a series of partial uncertainties, and its qualified estimate is a mandatory part of laboratory method validation. We distinguish between standard uncertainty, which is expressed as standard deviation (SD), combined uncertainty, which is a combination of partial uncertainties, and extended combined uncertainty, which is usually obtained by multiplying the combined uncertainty by two, the result corresponding approximately to a 95% confidence interval (CI) for normal distribution.

Acceptable measurement uncertainty

Each laboratory should define clinically acceptable measurement uncertainty for individual parameters.

The tolerable maximum uncertainty of measurement for the levels of biological substances in plasma was recently established in a published study² based on models proposed at the Milan Strategic Conference 2014. For ALT, it was set at ±6.98% and for bilirubin at ±15.7%. For alkaline phosphatase (ALP), the tolerable maximum uncertainty is ±4.5%, according to a 2017 source.³

Randomly, one Czech laboratory reports combined, respectively extended uncertainty determination of ALP in serum at 8, resp. 16%, ALT in serum at 4, resp. 8%, and bilirubin in serum at 6, resp. 12%.

Conclusion

Therefore, in clinical practice, it is necessary to keep in mind that results provided by the laboratory may differ from the actual value by the stated percentages up or down.

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Sources:
1. Budina M. Uncertainty of measurement results. Transfusion and Hematology Today 2018; 24 (3): 142−150. 
2. Braga F., Panteghini M. Performance specifications for measurement uncertainty of common biochemical measurands according to Milan models. Clin Chem Lab Med 2021 Mar 16, doi: 10.1515/cclm-2021-0170 [Online ahead of print]. 
3. Infusino I., Frusciante E., Braga F., Panteghini M. Progress and impact of enzyme measurement standardization. Clin Chem Lab Med 2017; 55 (3): 334−340, doi: 10.1515/cclm-2016-0661.