Subjects used to calculate pregnancy-specific reference intervals must exclude those who are overweight or obese

Dear Editor,

There is much confusion in the letter from Dockree et al. commenting on the reference intervals during normal pregnancy paper.^1,2 The values used in determining reference intervals are always selected from disease-free subjects. Adding people to these calculations with known characteristics that make a disease diagnosis likely is unwise and serves no legitimate clinical purpose. Body Mass Index (BMI) specific reference ranges are in truth indices of organ diseases. A disease is a deviation from the mean that constitutes a biological disadvantage to the holder. A raised BMI fulfils this criterion. Including known diseased subjects in identifying the normal or reference range in pregnancy trimesters is simply incorrect. Just because more than 50% of women in Britain and Ireland are overweight or obese is not a reason to pretend all are ‘normal’. Interpreting tests in the context of BMI is indeed important if the interpretation recognises that obesity is an illness. The WHO defines a BMI over 25 as overweight and a BMI over 30 as obese and both present a risk to health. The global burden of disease states that over 4 million people died from overweight or obesity in 2017. ³ The WHO is correct. Worldwide, most births occur in low income countries, and it is unlikely that procalcitonin or indeed CRP will be available and affordable as a routine in care in childbirth. Knowing the reference change values of common analytes through the perinatal period is more likely to aid interpretation. Total body water changes in pregnancy is a subject that should not be ignored.