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Laboratory protocol requires the use of two different QC specimens, QC-A and QC-B, during each run for Chemistry tests. The control limits are set at 95% confidence limits. At the completion of one run, you find that one control falls within the specified limits but the second control is out of control, falling between +2 SD and + 3 SD on your chart.
What is the probability that a QC specimen will yield a result of +/- 1 SD about the mean? • 66.6 % • What is the probability that a QC specimen will yield a result of +/- 2 SD about the mean? • 95% • What is the probability that a QC specimen will yield a result of > than +/- 2 SD about the mean? • 5% Use of a 95% confidence interval accepts that 5% of all analyses will fall outside of the allowable limits for a perfectly working instrument. Thus, 25/1,000 results will be greater than + 2 SD from the mean and 25/1,000 will be less than - 2 SD from the mean.
If one repeats the analysis immediately, what is the probability that the result will be within the +/- 2 SD range • For a single analysis, the probability of results falling within the current 95% target distribution remains the same. Thus, there is a 95% chance that a repeat analysis of the quality control sample, analyzed immediately following a random value which is outside the 2 SD limits, will yield a result falling within the +/- 2 SD range, if the instrument is working perfectly.
If an instrument has in fact, become mis-standardized because of electronic or reagent drift, and a QC specimen result is outside the +/- SD range, what would you expect the result of an immediate repeat analysis of the same quality control sample to be? • If the instrument is really miscalibrated, there will be a shift (bias) of the results towards a different population of results, i.e. one with a higher or lower mean value than the true target population for a perfectly calibrated instrument. Thus, an immediate repeat analysis of a quality control pool sample will have a 95% chance of falling within the population of results reflected by the new calibration, i.e. higher or lower than the normally accepted range.
One of two controls is within 2 SD and the other is between 2 SD and 3 SDWhat is the best course of action? • Rerun the control that is greater than 2 SD • If the repeat is within 2 SD report run • If repeat is still out Troubleshoot and rerun all specimens.
Given one result within the reference range and the other between two and three SDs from the mean, the technologist knows that the result greater than 2 SD could be due to random chance (2.5%) and that the instrument might be working perfectly. The analysis of the QC specimen that was out of range should be repeated. If it falls back within+/- 2 SD, one can assume that the out-of-range result was a random chance event and that the instrument is operating properly. The results should be sent out.
If both QC specimens are now within range, i.e. QC-A and the repeat QC-B, but both are near the upper or lower 2 SD limits, the technologist may want to recalibrate the instrument anyway since these results may reflect a small, but definite, shift. The technologist probably would send the patient results out, except for those borderline ones, which could be repeated.
If, on the other hand, the repeat analysis of the out-of-range qua1ity control pool is still out of range, there is a very high likelihood that there is a real shift in the calibration, and that this QC result reflects the new, biased population. The technologist should recalibrate and re-analyze all the patient samples up to the last within-range quality control result.
