Manufacturers’ Definitions of Performance
Advanced Instruments defines instrument performance using the following specifications1,2
Range: The upper and lower limits of nominal sample values that can be measured by the instrument.
Linearity: The closeness of agreement between the mean (average) value obtained for a series of results and the true value for the sample tested. It is typically expressed as a positive or negative percentage variation from a straight line. For example:
Linearity: Less than 1% of a straight line between the calibration points of 50 and 850 mOsm/kg H2O.
This means for any sample with an osmolality value between 50 and 850 mOsm/kg H2O, the calculated mean for a series of tested samples will vary by no more than ±1.0% from the nominal or true value of the sample. For example, if the sample value were 290 mOsm/kg H2O, the mean value of the reported results would be no greater than 292.9 mOsm/kg H2O or no less than 287.1 mOsm/kg H2O.
Repeatability: The variation of any individual test result with respect to the calculated mean (average) for a series of results obtained under the same conditions of measurement (i.e. same operator, short time interval, etc.). It is the Standard Deviation (S.D. or s) calculated for a series of results, and is typically expressed in mOsm/kg H2O or as a percentage. For example:
Repeatability: Standard deviation less than or equal to ±2 mOsm/kg H2O between 0 and 400 mOsm/kg H2O; less than or equal to ±0.5% between 400 and 2000 mOsm/kg H2O.
This means the standard deviation of a series of results for a sample whose mean value is within the range of 0 and 400 mOsm/kg H2O will vary no more than ±2 mOsm/kg H2O from the calculated mean. Similarly, the standard deviation of a series of results for a sample with a mean value within the range of 400 and 2000 mOsm/kg H2O will vary by no more than a mOsm/kg H2O amount equal to 0.5% of the calculated mean.
1 Other manufacturers may define the performance specifications for their instruments differently. It is important to consult the particular instrument datasheet and instructions for use, prior to analyzing laboratory data.
2 Refer to Determining the Acceptable Range of Results for an Application Using Specifications for the Instrument and a Calibrator/Control for specific examples of the calculation.