Intra-assay precision defined

Define the following terms instrument precision, injection precision, intra-assay precision, ruggedness, intermediate precision, and interlaboratory precision. 

ICH Q2A defines repeatability as the variability of the assay results under the same operating conditions over a short interval of time. Repeatability is also termed intra-assay precision. A reportable value of an assay is often the average of a specified number of replicate values, where the replicates are processed using the same sample preparation. Thus, it is not possible to obtain a true repeat of the assay s reportable value. Repeatability can be modeled as the within-assay variability of the replicates however, it should be noted that this precision is at the level of the replicate and not at the level of the reportable value. If the reportable value is an average of K replicates the variance of the reportable values will be less than that of the replicates by a factor of l/K (the standard deviation will be less by a factor of (1/ k ). 

The repeatability or intra-assay precision is defined as the precision of determinations performed in the same way and within a short space of time. Its determination is of particular interest in the analytical testing for the imiformity of the content of a measmed active ingredient (content uniformity test). 

In a protocol about collaborative studies [10] it is also considered what is called preliminary estimates of precision. Among these the protocol defines the total within-laboratory standard deviation . This includes both the within-run or intra-assay variation (= repeatability) and the between-run or inter-assay variation. The latter means that one has measured on different days and preferably has used different calibration curves. It can be considered as a within-laboratory reproducibility. These estimates can be determined prior to an interlaboratory method performance study. The total within-laboratory standard deviation may be estimated fi-om ruggedness trials [10]. 

Precision defines the scatter of repeated analysis, or the coefficient of variation of analytical results. Both intra-assay and inter-assay precision must be investigated. Intermediate precision describes the influence of different analysts, equipment, days and other intra-laboratory variabihty. Inter-laboratory comparison is also of interest in establishing the precision of the method. AU testing on accuracy and precision must be carried out by replicate analyses of a statistically relevant number of samples. Depending on the use of the method, it may be necessary to estabhsh both parameters over the measurable range, or in the case of content determination simply in the range of 80-120% of the nominal value. 

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