Precision and accuracy: concepts and assessment by method evaluation testing.

J O Westgard

doi:10.3109/10408368109106450

Home/Archive/PubMed

PubMed · 7016436

Precision and accuracy: concepts and assessment by method evaluation testing.

J O Westgard

View original source ↗Export metadata ↓

Abstract

Achieving precision and accuracy in routine clinical analyses is a complex task, requiring the identification, estimation, and elimination of sources of analytical error. This review first considers concepts of precision and accuracy, including discussions of the meaning of measurement process, analytical method, state of statistical control, precision, imprecision, accuracy, inaccuracy, systematic error, overall or total error, true value, traceability, and compatability. These concepts provide the basis upon which the performance of analytical methods can be evaluated. The second part of the review considers how precision and accuracy are assessed by the use of method evaluation experiments. The approach emphasizes the development of an evaluation protocol based on the analytical characteristics which represent the performance of the method. This includes discussions of the familiarization period; testing analytic range and linearity; testing precision by a replication experiment; testing accuracy by recovery, interference, and comparison of methods experiments; the selection of a comparative analytical method; the statistical analysis of method comparison data, including the interpretation of that data; the collaborative testing.

Explore related subjects

Aspartate Aminotransferases Chemistry, Clinical Clinical Laboratory Techniques Diagnostic Errors Evaluation Studies as Topic Humans Methods Quality Control Statistics as Topic Journal Article Review

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

J O Westgard. 1981. Precision and accuracy: concepts and assessment by method evaluation testing.. https://doi.org/10.3109/10408368109106450

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed1997

Transient-state kinetics of the reaction of aspartate aminotransferase with aspartate at low pH reveals dual routes in the enzyme-substrate association process.

In aspartate aminotransferase, the coenzyme pyridoxal 5'-phosphate forms a Schiff base with the epsilon-amino group of Lys258. The pH dependency of the steady-state kinetics of the overall reaction had indirectly suggested that the Schiff-base-unprotonated form of the enzyme (EL) is the active species that binds the monoanionic form of aspartate (SH+), the predominant species of the substrate in solution. In order to obtain direct information on the association process, we carried out transient-phase kinetics of the first half-reaction of the enzyme with aspartate at various pH. The disappearance of EL (lambdamax = 358 nm) was fast and independent of pH, but the disappearance of ELH+ (Schiff-base-protonated form, lambdamax = 430 nm) was slow and dependent on pH. At pH values below 6.8 and low concentrations of aspartate, the results could be interpreted to indicate that EL reacts rapidly with SH+ to form the pyridoxamine 5'-phosphate form of the enzyme (EM), and the reaction of ELH+ proceeds via the route ELH+ right arrow over left arrow EL right arrow over left arrow EM, where the first step was found to be rate limiting from the pH jump/drop study of the enzyme. At higher pH values, the rate of disappearance of ELH+ became larger than expected from the above scheme. This deviation became apparent with increasing pH, and could be excellently explained if we consider that it is due to the reaction of ELH+ with the dianionic form of aspartate (S). Thus, the formation of the Michaelis complex of aspartate aminotransferase and aspartate can proceed via two routes; route A is the association of EL with SH+ to form EL.SH+, which converts intramolecularly to ELH+.S, and route B is the association of ELH+ with S to form ELH+.S directly. ELH+.S is the prerequisite structure for further processing of the substrate by the enzyme. The reactions of EM and oxo acids yielded almost exclusively EL and SH+, and therefore route B does not seem to play an essential role in the overall reactions of the enzyme. Route B, however, may be important in the reaction mechanisms of other pyridoxal 5'-phosphate enzymes which have only the ELH+ form.

Aspartate Aminotransferases↗

PubMed1997

Simultaneous assay for aspartate aminotransferase and guanase in human serum by high-performance liquid chromatography.

A simple high-performance liquid chromatography (HPLC) assay for the simultaneous determination of guanase and aspartate aminotransferase (AST) activities in a single serum sample is described. The method is based on direct detection of enzymatically formed products xanthine and glutamate, respectively. The procedure is sensitive, precise (C.V. below 2% for guanase and 3% for AST), suitable for routine purposes and requires only 100 microliters of sample. Kinetic measurements have shown the guanase activity to have an apparent Michaelis constant of 24.5 microM and the AST activity of 11.1 and 0.18 mM for aspartate and oxoglutarate, respectively, at 37 degrees C in Tris-HCl buffer (pH 7.5).

Aspartate Aminotransferases↗

PubMed1997

Use of 1H-15N heteronuclear multiple-quantum coherence NMR spectroscopy to study the active site of aspartate aminotransferase.

Aspartate aminotransferase from Escherichia coli, an 88 kDa enzyme, was uniformly and selectively enriched with 15N and was studied by heteronuclear multiple-quantum coherence NMR spectroscopy in H2O. Good resolution was obtained for the downfield region (above 9.5 ppm chemical shift in the 1H dimension) for NH protons in the amide, indole, imidazole, and guanidinium group regions and several resonances were tentatively assigned. Two downfield resonances, at 12.6 and 11.36 ppm, appear to belong to oxygen- or sulfur-bound protons. The most downfield amide resonance at 11.78 ppm was assigned to the active site cysteine 192 whose peptide proton is 2.9 A away from the negatively charged carboxyl group of aspartate 199. Large downfield shifts (up to 1.15 ppm) of the indole NH resonance of the active site tryptophan 140 were observed upon binding of dicarboxylic inhibitors to the pyridoxal 5'-phosphate (PLP) form and of inorganic dianions to the pyridoxamine 5'-phosphate (PMP) form of the enzyme. We discuss these striking differences in the light of the available crystallographic data. Active sites of proteins, as well as specific inhibitory molecules, often contain negatively charged groups. These may be able to form hydrogen-bonds to NH groups and to shift the NH resonances downfield into a less crowded and therefore more readily observable region for many large proteins. Our approach, which makes use of both HMQC spectroscopy and NOE observations, should be widely applicable.

Aspartate Aminotransferases↗