Analytical Goals

Valid data are essential in making medical decisions, the most important concepts used in judging analytical performance :- 1)Analytical accuracy : agreement between the best estimate of a quantity and its true value 2)Analytical precision : agreement between replicates

Other two consideration are :- Analytical specificity : the ability of analytical methods to determine solely the components it purports to measure. Analytical sensitivity: the ability of an analytical method to detect small quantities of a measured components.

Setting analytical goal Analytical goals should be appropriate for intended medical use, and hence will vary in different clinical situation as well as among different analytes. although one intuitively assumes that a laboratory test is usually obtained for the purpose of the diagnosing disease, most laboratory tests are in fact obtained to monitor treatment.

there is a basic distinction in performance requirement for two purposes, for monitoring therapy or following the course of disease the concentration of one or more substance is compared with the value obtained previously on the same patients, in this case the day to day observation must be reliable, and precision or reproducibility is the prime concern for diagnosis on other hand, the concentration of one or more substance is compared with the established reference range or discrimination values to determine if the value for the subject.

Lab. Analysis : is used to screen for disease, an abnormal value may not lead to immediate therapy

Test to determine prognosis is an extension of the diagnosis into future, accuracy are not rigid as for the initial diagnosis. for example whether the serum prostatic acid phosphatase is 5 u or 6 u /l would make little difference in predicting the course of prostatic carcinoma but equal error in activity of 1u/l could be critical in the initial detection of this tumor

In emergency the timeliness of a report may be of such importance that a somewhat lower standard of performance in term of accuracy or precision is acceptable.

Goals for analytical accuracy Criteria for medical need in term of accuracy have seldom been define reasons are several laboratories have been in habit of defining the reference ranges believed applicable to their own local, there was little movement of patients around the country and consequently the little need for common standard for (the true value)

furthermore on practical scale, there was,for many analyte, no agreement on the true value among laboratories nor was their agreement on reference methods to obtain such a value. It cannot now be maintain that accuracy is less important than precision. patients are shifting geographical locations

frequently and there is a need for standardization of both analytical result and reference range among lab. Studies of patient or of groups of patients often extended over long period of time. Since there has been less experience in comparing bias of inter laboratory results than in comparing precision, there are few published analytical goals for accuracy of clinical lab. Tests,Gilbert in 1975 published accuracy goal as a percentage based on his experience with the evaluation of college of American pathologist CAP. survey results as shown in the table below they are stated to be approximation and to be nearly the same as coefficient of variation.

Gilbert approach reflect trust in the mean of values from many participating labs. As an approach to establishment of the true value for analyte.

C.V %Bias % Accuracy Goal % Definitive value Substance in serum mg/dlCalcium ug/dlIron mmol/lPotassium mmol/lSodium mg/dlCholesterol mg/dlGlucose mg/dlUrea mg/dlUrate mg/dlCreatinine

Analytical performance compare with analytical goals : With the development of definitive method for certain analyte in serum specimens, however it has been possible to assess accuracy of performance of clinical lab. For these analytes by applying the definitive methods to serum pools used national wide survey programs. Definitive methods usually employ isotope dilution /mass spectrometry and furnish a value that is close to two value for the analyt in serum matrix as is obtainable with current technology, two such studies are shown in table 2, Gilbert compare the mean of the results obtained by CAP survey participant with definitive value for 7 analytes assigned by the US national Bureau of standard.

Analytical Precision C.V % Precision Required for Medical use C.V % Mean ValueAnalyte mg/dlGlucose mmol/lK mmol/lNa mg/dlT.G mg/dlUrate g/dlALB I.U/LALK.Ph I.U/LGPT I.U/LGOT o mg/dlT.Bil mg/dlChol.

Accuracy performance as judged by clinical lab. Values for bias in table 2 was within 2% for 9 of the 13 substances tested at the level shown. For urea it about 3% at extremes of the ranges of concentrated tested. The bias in the average results of survey for many frequently assayed substance is seen to be small less than CV of the assay this also indicate that the mean value generated in a large number of participating lab. For a common pool is a reasonable estimate of true value for these substance, this conclusion cannot applied as yet to other analyt particularly enzyme for which reaction condition vary among labs.