The Normalisation Fallacy

Posted by Jeremy Windsor on Sep 11, 2020

"The Normalisation Fallacy" 

A tendency to view abnormal laboratory values and physiological parameters as targets for normalization.

Isn't it tempting to treat numbers in medicine? In critical care we're busy manipulating all sorts of figures - from ventilator settings and cardiac measurements, to nutritional requirements and a host of different blood results. But are we actually helping? Could we be doing harm? Take for instance, the measurement of vitamin D in critical care. Vitamin D deficiency is common and there is a growing feeling that treatment might make a real difference to patients. However, the recently published VIOLET trial goes some way to discourage this practise. Using a well established definition of vitamin D deficiency (<20ng/mL) and treatment (a single dose of 540,000iu) the study set out to see if treating vitamin D deficiency would lead to lower mortality in the critically ill. 

Vitamin D deficiency is implicated in a range of different medical conditions ranging from asthma and hypertension to fractures and a range of mental health problems. But is it something that should be treated in critical care?

More than a 1000 critical care patients with vitamin D deficiency were randomly assigned to receive either vitamin D or placebo. As expected, Vitamin D levels rose significantly in those given the supplement (46.9 vs 11.5ng/ml) however outcomes were unaffected. With treatment, the 90 day mortality was 23.5% (125/531) compared to 20.6% (109/528) in the placebo group. There were no other clinical differences between the 2 groups and importantly, the severity of vitamin D deficiency at baseline did not affect outcome. Put simply, even those with the lowest Vitamin D concentrations did not see any benefit by taking the supplement.

When faced with an abnormal measurement in medicine it can mean - (1) it's bad and needs treating, (2) it's good and should be left well alone or (3) it's simply a coincidence (an "epiphenomenon"). From a critical care point of view, it looks like Vitamin D deficiency is something that doesn't need treatment and might simply be a coincidence. A good example of the "Normalisation Fallacy"!

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