To judge a finding in a patient, be it body weight or height, a blood test like white cell count or serum potassium etc. we usually compare it with so called ‘normal’ values (better called reference values or range). These values are obtained by studying the frequency distribution of the finding (variable) in question in a sample of healthy people and determining its mean and standard deviation (SD). The normal range is taken as the range between + 2 standard deviations and -2 standard deviations.
(Normal frequency distribution curve with number of standard deviations on the horizontal axis)
We know from statistics that this will include 95% of people which means that 5% of healthy people have values outside the normal range. We have to remember then that if a patient’s measurement is slightly outside the normal range it still can be normal. Conversely if it is within the normal range it still can be abnormal for that particular patient because it is significantly different from his usual figure. For example a patient whose white cells count is 4000/c.mm. in normal circumstances the number may rise to 8000/c.mm. when he develops an infection. It is still in the normal range though it is raised compared to his count during health.
Like every thing else in medical practice data have not to be taken blindly and in isolation but have to be interpreted with caution taking all other findings to form an overall picture of the situation.