Clinical PotpourriExploring the impact of using measured or estimated values for height and weight on the relationship between BMI and acute hospital mortality
Introduction
The relationship between body mass index (BMI) and mortality in critical care populations is unclear [1], [2], [3]. While most studies agree that being very underweight, a very low BMI, is disadvantageous, the relationship of being overweight or obese, a high or very high BMI, varies across studies. The variation seen across studies may be due to the degree of adjustment for other confounders and/or the precision of the measurements of height and weight used to calculate BMI.
In the UK, the National Institute for Health and Care Excellence (NICE) guideline specifies that all patients have their height and weight measured on admission to hospital [4], however, these measurements are often not performed [5]. In the ICU setting, the condition of the patient, lack of suitable equipment or high workload may render measurements of height and weight challenging [6]. Most papers discussing the association between BMI and mortality in this setting do not report whether assessments of height and weight were measured or estimated [7].
The Intensive Care National Audit & Research Centre (ICNARC) coordinates the national clinical audit for adult critical care, the Case Mix Programme (CMP), in England, Wales and Northern Ireland. As part of data collection, height and weight are recorded along with whether these values are measured or estimated. The aim of this study was to explore the impact of using measured or estimated values for height and weight on the relationship between BMI and acute hospital mortality.
Section snippets
Case Mix Programme Database
Case mix and outcome data for the Case Mix Programme are collected on consecutive admissions to participating ICUs to precise rules and definitions, by trained data collectors and undergo extensive local and central validation. Details of the data collection and validation have been reported previously [8]. The Case Mix Programme Database has been independently assessed to be of high quality [8] and support for the collection and use of patient-identifiable data without consent has been
Case mix and outcomes
The inclusion criteria were met by 690,405 admissions to 219 ICUs (Fig. 1). Most patients had estimates of height and/or weight rather than actual measurements (59.8%). Height was estimated more often than weight (Table 1).
Patients with both height and weight measured had lower physiology scores, were more likely to have been admitted to ICU following elective surgery and were less likely to have received CPR. They were more likely to have been living independently and their reasons for
Discussion
Methods for estimating height and weight range from formalised algorithms based on easily obtainable measures to subjective guesses, but all of these techniques are prone to variation and bias [15], [16], [17], [18], [19], [20], [21].
Most ICU patients do not have their height and weight measured but practice varies considerably both across ICUs and according to patient characteristics. Our data confirm the previously reported reverse-J-shaped relationship between BMI and mortality with the
Conclusion
In conclusion, our study demonstrates that height and weight are estimated for a substantial proportion of patients admitted to UK ICUs. For epidemiological purposes, heights and weights estimated by the currently used techniques are valid approximations of the actual values.
The following are the supplementary data related to this article.
Conflicts of interest
On behalf of all authors the corresponding author states that there are no conflicts of interest.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgements
We are very grateful to Amber Tiffany Rucinski for help with the literature searching related to this work.
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