Variation in common laboratory test results caused by ambient temperature

Laboratory tests measure important aspects of physiology, but their results also vary for idiosyncratic reasons. We explore an underappreciated source of variation: ambient temperature on the day blood is drawn.

In a sample of 4,877,039 individuals between 2009-2015, we model 215,234,179 test results as a function of temperature, controlling for individual and city-week fixed effects. This measures how day-to-day temperature fluctuations affect results over and above the individual's mean values, and seasonal variation.

51 of 75 assays are significantly affected by temperature, including measures of kidney function (increased creatinine, urea nitrogen, and urine specific gravity), cellular blood components (decreased neutrophils, erythrocytes, and platelets), and lipids (increased high-density lipoprotein [HDL] and decreased total cholesterol, triglycerides, and low-density lipoprotein [LDL]). These small, day-to-day fluctuations are unlikely to correlate with long-term physiological trends; for example, lipid panels checked on cooler days look lower risk, but these short-term changes probably do not reflect stable changes in cardiovascular risk. Nonetheless, doctors appear to treat these individuals differently. We observe 9.7% fewer statin prescriptions for individuals checked on the coolest versus the warmest days (-0.42% versus baseline of 4.34%, p < 0.001).

Ambient temperature affects the results of many laboratory tests. These distortions, in turn, affect medical decision-making. Statistical adjustment in reporting is feasible and could limit undesired temperature-driven variability.

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