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Within Run Precision Within run replication, or precision, must be evaluated and accepted before proceeding with more comprehensive studies. Precision indicates how well a method or instrument gives the same answer when a single sample is tested repeatedly. Precision measures the random error of a method, which is the scatter in the data. Within run precision provides an optimistic estimate of the expected daily performance of a method or instrument since there has been minimal opportunity for operating conditions to change during the analytical run. Both pooled patient sera and quality control samples can be used for precision studies. Concentration of the analyte in the patient pool should ideally be slightly above the upper limit of the reference range. Concentration of the analyte in the quality control samples should be as close as possible to the upper and lower medical decision points (usually the upper and lower reference limits). Each quality control or patient sample is run at least 20 times, with at least 2 reagent lots. Mean, standard deviation, and coefficient of variation are calculated for each level. The greater the imprecision, the larger the standard deviation will be. The higher the standard deviation, the greater the percentage of the mean it becomes and the higher the %CV (CV = standard deviation/mean x 100). If the instrument or method has good precision, 95% of values should fall within 2 standard deviations of the mean. If more than 1 of the 20 results fall outside of 2 standard deviations, the method should be rejected. Acceptable CVs need to be defined for each analyte based on medical significance. Generally, precision should be equal to or less than one half of the within subject biological variation (see Appendix A). Between Run Precision Between run replication is the best indicator of a method's overall precision because it measures the amount of random error inherent in the method from day to day and takes into account variable factors such as changes in operators, reagents and ambient operating conditions. Between run precision should be combined with method comparison studies. Quality control samples and a pool of patient samples can be used. Concentration of the analyte in the patient pool should be slightly above the upper limit of the reference range. Concentration of the analyte in quality control samples should be as close as possible to the upper and lower medical decision points (usually the reference limits). Between run replication should be evaluated over at least 20 days or runs using at least 2 reagent lots. The mean, standard deviation, and coefficient of variation are calculated for each level. The standard deviation obtained during day to day replication studies is expected to be greater than the standard deviation of within run studies. The maximum allowable between run standard deviation is a matter of judgment. Generally, it should be less than total allowable error (see Appendix B). |
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