The Use of Laboratory Test Results in Health Care Management

The Reference Range

Laboratory results are conventionally compared with established physiologically normal values, such values being often derived from text books. For any individual, the ideal reference value for a test result would be that obtained when that individual is healthy. Test results for the normal and abnormal can overlap, and a value within the accepted normal range may be pathological for a particular subject. For this reason, more recently, the concept of normal ranges has been replaced by the concept of reference ranges and test results of patients have been compared with the reference ranges. Ideally, each laboratory should establish its own data bank of reference values and reference ranges.

A reference individual is one selected using defined criteria and picked from a population that includes all individuals who meet those criteria. A reference sample is a number of such reference individuals. Reference values are test results obtained by testing the reference sample or population. They can be subject to statistical analysis, and they will show a certain distribution. Leaving out the lowest 2.5 percent and the highest 2.5 percent at either end of the observed range, the central 95% of the observed values is known as the reference interval (Bain, 1995). Reference ranges for a particular test for a select population are determined from groups of comparable subjects assumed to be representative of the population being dealt with, namely a reference population, of defined criteria such as age, sex, non-smokers, non-alcoholics, non- pregnant (if female), not on regular drugs and free from chronic disease. The technique and timing of sample collection and posture of the subject at the time the sample is taken must be standardized. Whether the subject is ambulant or resting in bed also may affect the test results. Samples should be collected at the same time of the day, in fasting state, with the subject in standardized position i.e. either seated or recumbent with the arm semi-flexed and with minimum stasis (Lewis et al, 2001).