A scheme for the evaluation of methods in clinical chemistry with particular application to those measuring enzyme activities. Part I: general considerations

Longitudinal distribution of mucosal enzymes in the human large bowel

The distribution of a series of mucosal enzymes along the large bowel was studied by analysis of homogenized biopsy specimens from five different segments, obtained from 20 control patients. The activities varied significantly between the segments for the membrane enzymes lactase (p less than 0.005), alkaline phosphatase (p less than 0.0005), leucyl-beta-naphthylamidase (p less than 0.0001), and 5'-nucleotidase (p less than 0.001) and the mitochondrial enzyme monoamine oxidase (p less than 0.0005) when tested by analysis of variance modified for repeated measurements. When paired comparisons between segments were evaluated, the enzyme activities of the proximal large bowel were significantly higher than those of distal segments. The levels of sucrase, neutral-alpha-glucosidase, gamma-glutamyltransferase, and lysosomal enzymes remained unchanged throughout the large intestine, as did the protein to DNA ratio. The results are compatible with the theory that different segments of the large bowel have different functions.

Use of an enzyme standard in the measurement of enzymes in a Clinicon reaction rate analyser

Measurement of enzyme activity in a Clinicon reaction rate analyser was carried out in two ways: one method was based on absolute calibration and the other used an enzyme standard which involved adjustment of results according to the value for the standard obtained in each batch. Batch calibration significantly improved long-term precision and reduced differences between results obtained in different instruments.

Assessment of colorimetric enzymatic determination of triglyceride, by manual and centrifugal analyzer techniques, and comparison with a CDC standardized method

We describe the assessment of an enzymatic method, with colorimetric detection, for the determination of triglyceride. Two commercial single vial reagent sets were evaluated, by manual and centrifugal analyzer techniques, and compared to a method which had attained standardized status from the Center for Disease Control (CDC). Precision and accuracy obtained on analysis of CDC reference materials, and precision obtained on analysis of plasma samples, did not fulfill the criteria of CDC. The precision met the criteria of the College of American Pathologists based upon intra-individual variation. Good correlation between the reagent set method and a continuous-flow method with prior extraction was evident but proportional and constant biases were found. The method would be suitable for routine use particularly if a centrifugal analyzer was employed. If a manual method was used it is recommended that blanks be performed on samples. The method had acceptable linearity. Use of a constant relationship between concentration and absorbance did not lead to improved performance characteristics.

Precision and accuracy: concepts and assessment by method evaluation testing

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.