Total Probable Variations
In situations where several independent factors combine to comprise the total variation possible for a system, the calculation that numerically adds the limits of each contributor (assuming that all exist at their maximum possible allowance), can produce limits, which exceed the amount of variation experienced during actual testing. In this case, a more realistic calculation of the total expected variation in system performance can be obtained from a root-mean-square (RMS) calculation. Although this method is most appropriate when the number of contributors is large, the calculation for the combination of the calibrator/control and the osmometer would be:
Using this result, the acceptable range of results for any individual test result within the data set are:
Lower range limit: 290 - 4.05 = 285.95 mOsm/kg H2O
Upper range limit: 290 + 4.05 = 294.05 mOsm/kg H2O.
Notes:
1. The above example does not include any effects from:
a. Improper instrument calibration
b. Improper pipeting technique (sample-to-sample volume variation, trapped air bubbles, etc.)
c. Instrument operation at a temperature significantly different from the calibration temperature
d. Long-term instrument drift since last calibration.
These factors can add additional variation to the overall system performance and further broaden the range of expected results.
2. Although osmolality is determined by monitoring the temperature of a sample being frozen through a highly controlled and repeatable process, this freezing process is nevertheless a natural process with its own level of randomness. This can occasionally produce test results with unexpectedly high or low values that exceed the acceptable range as determined above. On these rare occasions, the sample should be retested or the test repeated by testing a fresh sample to confirm or reject the prior result.
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