•Analysis of a dataset of nearly 5 million and over 200 million laboratory test results
•Ambient temperature on the day an individual’s blood was drawn affected the test result
•Affected assays included renal function tests, complete blood count, and lipid panels
•Doctors prescribed fewer statins to individuals whose lipids were checked on colder days.
Context and significance
Doctors use laboratory tests to measure many important aspects of physiology. But test results can also vary for arbitrary reasons, like ambient temperature on the day of the test. In a United States-based sample of nearly 5 million people and 215 million test results, day-to-day temperature fluctuations affected the results of some of the most commonly used laboratory tests in medicine, including lipids and red and white blood cells.
Although the changes were small, they did influence doctors’ medical decisions; individuals whose lipid panels were checked on colder days appeared to be at lower risk for cardiovascular disease, leading to a 10% lower likelihood of being prescribed a statin. Understanding these changes means that laboratories could correct some results based on 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.