Analytical variation
The analytical variation of the research method used has a great influence on the results of laboratory studies. The main criteria for evaluating the method of investigation are accuracy, reproducibility, specificity, sensitivity.
■ Accuracy characterizes the validity of the method in determining the exact value( concentration) of a substance. For example, the systematic difference between the results of the determination of sodium in one sample of more than 3 mmol / l is considered unacceptable. On the other hand, the more significant difference between the concentrations of certain hormones determined by ELISA with different AT is considered acceptable, since the use of different AT drugs gives different matrix effects. For this reason, various intervals of reference values are established for individual immuno-enzymatic methods for determining hormones.
■ The reproducibility of the method is assessed by measuring the concentration of a substance in the same sample several times on the same day and in the same series of samples. The next day they make the same measurements with the same sample. Usually the deviations in the measurements are subject to Gaussian law, which indicates the stability of the method. For each series of measurements, calculate the average value( Xcp).Then find the difference between the value of each measurement and this mean and calculate the standard deviation( S) and the coefficient of variation( V).Determine the coefficient of variation on other days, and if it does not exceed 5%, then the method of investigation is considered adequate. For enzymes, V can reach 10%.It is the responsibility of each laboratory to verify the reproducibility of the methods, which are estimated from the standard deviation( SD).For example, reproducibility in determining the concentration of total cholesterol in the blood serum in a good laboratory is usually on the average +0.13 mmol / l. It is known that 95% confidence interval is + 2SD, which in this case corresponds to 0.26 mmol / l. Thus, each result is considered true if it is within these limits( +0.52 mmol / l).Thus, the concentration of total cholesterol in the blood serum is 5.18 mmol / l, meaning that the true value is between 4.92 and 5.44 mmol / l.
■ Specificity - the ability of a method to measure only the component for which it is intended. To assess the analytical specificity, impurities are used which, based on the chemical structure, are representative representatives of those groups of substances that, physiologically, have
practical significance. To a greater extent this applies to drugs that can cause chemical interference during the analysis. Low specificity and the influence of interference leads to an incorrect result( not to be confused with the specificity of the method for pathology).
■ The analytical sensitivity of the method is the smallest amount of substance( the lowest concentration) that can be detected by this method. This concept should be distinguished from the sensitivity of the method with respect to the detection of a particular pathology. When choosing the method of research, it is necessary to pay close attention to the analytical sensitivity of the method, since the quality of research results depends on this. For example, Order No. 282 of the Ministry of Health of the Russian Federation of 28.09.98 "On the use of immune-enzyme systems for the detection of superficial hepatitis B virus( HBsAg) and anti-hepatitis C virus( anti-HCV) in human sera"systems for the detection of HBsAg, the sensitivity of which exceeds 0.5 ng / ml, as well as test systems for the detection of anti-HCV that do not contain proteins encoded by the NS3 RNA region of hepatitis C virus. The use of test systems for the detection of HBsAg with sensitivityabove 0.5 ng / ml and not containing protein in its compositionin, encoded by the NS3 zone of RNA of hepatitis C virus, leads to the fact that viral hepatitis B and C in a number of patients are not diagnosed.
The analytical variation, depending on the methods used and the conditions of their implementation, extends the limits of normal laboratory indicators and this limits the possibility of laboratory tests to distinguish between health and disease. Therefore, laboratory specialists should strive to reduce the analytical variation. In Table.the maximum permissible limits of the analytical variation( spread) of the analyzed components are given.
the values of the permissible analytical variation( V) are considered as mean indicative values. These variations, given for leukocytes and erythrocytes, refer to the calculation of cellular elements by manual methods, with the use of hematological analyzers, the coefficient of analytic variation for leukocytes is 1-3%, for erythrocytes 1-2%, platelets 2-4% [Elevitch FR etal., 1987;Koepke, J. A., 1993].
Confirmation that the analytical variation of the method can have a significant effect on the results of the study is provided in Table.data of 95% confidence interval when calculating the leukocyte blood formula, obtained on the basis of statistical analysis.
Thus, when evaluating the results of laboratory studies, the physician should take into account the variety of factors affecting the results, know the analytical reliability of laboratory methods of research, that is, be sure of the accuracy of information obtained with their help about the relevant components of the biomaterial. Knowledge of the degree of variability of the results of studies is important for comparison with biological variability, as well as for comparison with clinically significant shifts in laboratory indicators. These criteria are determined when developing methods, indicate in their description, and if necessary, the laboratory doctor should inform the clinician about it.
Table Maximum permissible limits for analytical variation( variation) for various components( Compendium of Laboratory Diagnostic Methods, 1984 6.1. CMLD)
The clinician needs to consider the following facts at the technical and biological levels of laboratory results evaluation.
■ Comparison of the result of the analysis with the reference range of the corresponding values indicates only the probability of compliance or non-compliance of this result with the norm.
■ There are physiological differences in normal values and physiological variations from day to day( biological variation).
■ There are small, technical differences in the results of analyzes obtained on different days( analytical variation of the method).
■ Reference ranges can vary with different laboratory methods.
■ The change in the content of the test component can be nonspecific and not associated with a primary metabolic disorder of this component( interference, hemolysis, lipemia, drug use, etc.).
■ There are random variations, the causes of which are not currently clear, but they should be taken into account when interpreting the results of repeated analyzes;for example, the daily variations in the concentration of iron in the blood are very large and can make it difficult to identify the patterns of changes in this component.
■ When examining plasma or blood serum, information on extracellular concentrations of the test components is obtained. These concentrations depend on the amount of water in the extracellular space with respect to the amount of the component being measured and may not always reflect the intracellular level of the test substances.