Calculation of indirect reference intervals of plasma lipase activity of adults from existing laboratory data based on the Reference Limit Estimator integrated in the OPUS::L information system

Objectives

Most laboratories have difficulties to determine their own reference intervals for the diagnostic evaluation of patient results by direct methods. Therefore, data is often just taken from the literature or package inserts of the analytical tests.

Methods

The section on Reference Limits of the German Society for Clinical Chemistry and Laboratory Medicine (DGKL) first uploaded the Reference Limit Estimator (RLE) as an R-program with MS Excel-interface on the DGKL home page and now this tool is implemented in the commercial Laboratory Information System OPUS::L (OSM AG Essen, Germany). We used this OPUS::L “Population specific Reference Limits” tool online with our laboratory database. First calculations were done using the example of lipase.

Results

The manufacturer’s original reference interval for lipase 12–53 U/L (adults) was changed to age dependent upper reference limits of <41 U/L (<20 years), <60 U/L (20–80 years) and <70 U/L (>80 years).

Conclusions

By means of the OPUS::L “Population specific Reference Limits” tool we were able to establish our laborarotry specific reference interval for plasma lipase activity. The new reference limits helped to solve an old problem of implausible low elevated lipase values.

The influence of sampling time on indirect reference limits, decision limits, and the estimation of biological variation of random plasma glucose concentrations

Objectives

Plasma glucose concentrations exhibit a pronounced daytime-dependent variation. The oscillations responsible for this are currently not considered in the determination of reference limits (RL) and decision limits.

Methods

We characterized the daily variation inherent in large-scale laboratory data from two different university hospitals (site 1 n=513,682, site 2 n=204,001). Continuous and distinct RL for daytime and night were estimated. Diurnal characteristics of glucose concentrations were further investigated by quantile regression analyses introducing age and cosinor-functions as predictors in the model.

Results

Diurnal variations expressed as amplitude/Midline Estimating Statistic of Rhythm (MESOR) ratio, averaged 7.7% (range 5.9–9.3%). The amplitude of glucose levels decreased with increasing concentrations. Between 06:00 and 10:00 h an average decrease of 4% has to be considered. Nocturnal glucose samples accounted for only 5% of the total amount but contributed to 19.5% of all findings over 11.1 mmol/L. Partitioning of RL between day and night is merely justified for the upper reference limit. The nocturnal upper RLs for both genders differed from those obtained during the day by 11.0 and 10.6% at site 1 and by 7.6 and 7.5% at site 2.

Conclusions

We conclude that indirect approaches to estimate upper RL of random plasma glucose concentrations require stratification concerning the time of sample collection.

Review of potentials and limitations of indirect approaches for estimating reference limits/intervals of quantitative procedures in laboratory medicine

Reference intervals (RIs) can be determined by direct and indirect procedures. Both approaches identify a reference population from which the RIs are defined. The crucial difference between direct and indirect methods is that direct methods select particular individuals after individual anamnesis and medical examination have confirmed the absence of pathological conditions. These individuals form a reference subpopulation. Indirect methods select a reference subpopulation in which the individuals are not identified. They isolate a reference population from a mixed population of patients with pathological and non-pathological conditions by statistical reasoning.

At present, the direct procedure internationally recommended is the “gold standard”. It has, however, the disadvantage of high expenses which cannot easily be afforded by most medical laboratories. Therefore, laboratories adopt RIs established by direct methods from external sources requiring a high responsibility for transference problems which are usually neglected by most laboratories. These difficulties can be overcome by indirect procedures which can easily be performed by most laboratories without causing economic problems.

The present review focuses on indirect approaches. Various procedures are presented with their benefits and limitations. Preliminary simulation studies indicate that more recently developed concepts are superior to older approaches.