Reference ranges were mentioned earlier. The reference range for any particular analyte can be obtained by measuring it in healthy individuals from a representative sample of the local population. Most laboratories use healthy blood donors. It may be important to know the normal range
in adults, children, males, females, particular ethnic groups, or pregnant or postmenopausal women. However, since no analytical method is 100% accurate and since individuals vary, care must be taken when establishing a reference range. To determine the reference range, the values for the measured analyte are plotted against their frequency in the selected population. In most values clustered around the center as seen in Figure 1.20. The mean ( x ) and standard deviation (SD) can be determined from these data.
In general, the reference range is taken to be between two standard deviations either side of the mean. This will cover 95% of the values obtained for the selected sample (provided the curve is Gaussian). The 95% reference range was selected as this minimizes any overlap between the results for a healthy population and those for a population with the disease. However, choosing a 95% range is one of the major limitations of reference ranges, since 5% of healthy individuals will, by definition, give results that are outside these values. Thus a test result outside the reference range does not necessarily imply that the individual is ill although it does indicate that there is a greater likelihood of the presence of disease.
The profile of test results for some substances, for example serum bilirubin, does not give a Gaussian curve but shows a skewed distribution. This skewed distribution can be transformed mathematically to a Gaussian distribution and a normalized reference range calculated.
The values of a number of analytes, such as serum iron and alkaline phosphatase vary with the age or sex of the patient. In such cases, age- and sex-matched reference ranges are required. When interpreting results for a particular patient, the ideal reference value would be obtained from the same patient before their illness and this is sometimes possible. For example, the concentrations of electrolytes in serum can be measured in a patient before an operation for comparison with those obtained postoperatively. However, in most cases the results for the patient before they became ill are not available.