Mendelian randomization: estimation of inpatient hospital costs attributable to obesity

Background Mendelian Randomization is a type of instrumental variable (IV) analysis that uses inherited genetic variants as instruments to estimate causal effects attributable to genetic factors. This study aims to estimate the impact of obesity on annual inpatient healthcare costs in the UK using linked data from the UK Biobank and Hospital Episode Statistics (HES). Methods UK Biobank data for 482,127 subjects was linked with HES inpatient admission records, and costs were assigned to episodes of care. A two-stage least squares (TSLS) IV model and a TSLS two-part cost model were compared to a naive regression of inpatient healthcare costs on body mass index (BMI). Results The naive analysis of annual cost on continuous BMI predicted an annual cost of 21.61 pound [95% CI 20.33 pound - 22.89] pound greater cost per unit increase in BMI. The TSLS IV model predicted an annual cost of 14.36 pound [95% CI 0.31 pound - 28.42] pound greater cost per unit increase in BMI. Modelled with a binary obesity variable, the naive analysis predicted that obese subjects incurred 205.53 pound [95% CI 191.45 pound - 219.60] pound greater costs than non-obese subjects. The TSLS model predicted a cost 201.58 pound [95% CI 4.32 pound - 398.84] pound greater for obese subjects compared to non-obese subjects. Conclusions The IV models provide evidence for a causal relationship between obesity and higher inpatient healthcare costs. Compared to the naive models, the binary IV model found a slightly smaller marginal effect of obesity, and the continuous IV model found a slightly smaller marginal effect of a single unit increase in BMI.