The use of ‘normal’ values

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.

Changing medicine

Doctors who practice medicine for many years are struck between now and then by changes that turn their previous beliefs upside down. I say this with two examples in mind:

1. The use of antibiotics in the treatment of peptic ulcer: Before the discovery of Helicobacter pylori and then establishing its relationship to the development of peptic ulcer no one would have been stupid enough to consider antibiotics when confronted with a patient with peptic ulcer. In fact mentioning it in an examination would have been a sufficient reason to fail the candidate in the opinion of many examiners.
2. The use of beta blockers in the treatment of heart failure: Not many years ago we considered these drugs contraindicated in the presence of heart failure because they depress myocardial contractility. Now they are considered an essential part of the treatment of heart failure due to systolic dysfunction except when there is some strong contraindication like bronchial asthma.

 It makes one wonder which of the ideas that we consider now established facts will turn to be fallacies in the future and which of the forbidden practices nowadays will become established practices tomorrow. As they say, change is the only constant in medicine!