What is an appropriate crossover study for a new lot of QC materials for a test that we don’t run every day? If we aren’t running tests every day, do we need to do so in order to complete a 20-day study? When can we run an abbreviated study and what’s an acceptable amount?
A crossover study is performed before the current lot of quality control (QC) materials expires, or before test sites run out of QC materials. The new lot is run in parallel with the current lot in order to establish new control limits. In order to ensure that the new control values reflect changes in personnel, reagents, calibrators, testing conditions, maintenance, and other factors that influence QC results, statisticians typically recommend that at least 20 values for estimating the means and standard deviations (SDs) of the new lots of QC materials should be collected over 10 or 20 days.
When this is not possible—particularly for materials with short outdates or when current lots are in short supply—test sites can evaluate QC materials over a shorter period of time. A guideline from the Clinical Laboratory and Standards Institute, Statistical Quality Control for Quantitative Measurement Procedures: Principles and Definitions (CLSI Guideline C24-Ed4) , suggests one provisional approach for establishing the new mean value by making no more than four control measurements per day for 5 days. The coefficient of variance (CV) from the old lot number having a mean similar to the mean of the new material can then be used to establish new control limits.
However, what many forget is that once sufficient control data are collected over a longer period of time, a new mean and SD for the new control materials should be determined, and control limits reevaluated and adjusted accordingly.
Resources: http://webstore.ansi.org/RecordDetail.aspx?sku=CLSI+C24-Ed4
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