The normal range: it is not normal and it is not a range

Correspondence to Dr Martin Brunel Whyte, Department of Medicine, King’s College Hospital NHS Foundation Trust, London SE5 9RS, UK; [email protected]

The ‘Choose Wisely’ campaign was introduced by the Academy of Medical Royal Colleges in 2016 with a view to encouraging a dialogue between clinician and patient regarding the practice of evidence-based treatment regimens.¹ Clinicians have access to a vast array of investigations and interpretation of the results of these tests is central to our role. A fundamental question that the patient wants answered is ‘are my tests normal?’

But what is meant by ‘normal’?

With every test result, the clinical laboratory will provide comparator value(s) to help the clinician place the result in context. The comparator values are often referred to as the normal range. A frequent occurrence is for the results within this interval to be colour coded, for instance black if the result is within the range and red when outside of it. This reinforces the concept of a result having a binary quality: normal or abnormal.

If we say that a blood result is normal, a number of inferences of dissimilar nature could be put on this. The difficulty was neatly captured by the philosopher Edmond Murphy in 1960s (table 1).² In the clinical setting, our experience is that the term is frequently used to imply that the patient has no physiological derangement and/or that the distribution follows a Gaussian distribution.

Table 1

Interpretations of ‘normal’ (modified from Murphy, 1966²)

	Conceptions of normal	Suggested alternatives
1	Determined statistically	Gaussian
2	Most representative of its class	Average, median, modal
3	Most commonly encountered	Habitual
4	Wild-type: most suited to survival & reproduction	Fittest
5	Harmless ‘carrying no penalty’	Innocuous/harmless
6	Most often aspired to	Conventional
7	The most perfect of its class	Ideal

If it is assumed that a ‘normal’ result has no pathophysiological derangement, the corollary would be that a result outside this limit would signal a disease state. This seems an arbitrary dichotomous interpretation. As the American psychiatrist, Theodore Rubin, put it ‘health may be considered a relative and not an absolute state’. Health may be conceived differently in different countries, or in the same country at different times, or even in the same individual at different...