Abstract
Laboratory data are frequently collected throughout the care of critically ill patients. Currently, these data are interpreted by comparison with values from healthy outpatient volunteers. Whether this is the most useful comparison has yet to be demonstrated. To understand how the distribution of intensive care unit (ICU) laboratory values differs from the reference range, and how these distributions are related to patient outcomes. Cross-sectional study of a large critical care database, the Medical Information Mart for Intensive Care database, from January 1, 2001, to October 31, 2012. The database is collected from ICU data from a large tertiary medical center in Boston, Massachusetts. The data are collected from medical, cardiac, neurologic, and surgical ICUs. All patients in the database from all ICUs for 2001 to 2012 were included. Common laboratory measurements over the time window of interest were sampled. The analysis was conducted from March to June 2017. The overlapping coefficient and Cohen standardized mean difference between distributions were calculated, and kernel density estimate visualizations for the association between laboratory values and the probability of death or quartile of ICU length of stay were created. Among 38 605 patients in the ICU (21 852 [56.6%] male; mean [SD] age, 74.5 [55.1] years), 8878 (23%) had the best outcome (ICU survival, shortest quartile length of stay) and 3090 (8%) had the worst outcome (ICU nonsurvival). Distribution curves based on ICU data differed significantly from the hospital standard range (mean [SD] overlapping coefficient, 0.51 [0.32-0.69]). All laboratory values for the best outcome group differed significantly from those in the worst outcome group. Both the best and worst outcome group curves revealed little overlap with and marked divergence from the reference range. The standard reference ranges obtained from healthy volunteers differ from the analogous range generated from data from patients in intensive care. Laboratory data interpretation may benefit from greater consideration of clinically contextual and outcomes-related factors.
Highlights
The standard reference ranges obtained from healthy volunteers differ from the analogous range generated from data from patients in intensive care
The use of a standard hospital laboratory value reference interval for data from patients in the intensive care unit (ICU) may be problematic in several respects
With probability density curves as graphical displays and a statistical approach appropriate for the comparison of these graphical results, we found that the probability distribution generated by data from patients in the ICU differed significantly from the standard reference interval
Summary
The laboratory reference intervals that define normal values are established by sampling healthy outpatients; these intervals are used to define abnormal values in patients in the intensive care unit (ICU).[1] whether this is a valid or useful comparison has never been shown experimentally, and what values should count as abnormal in the critically ill population is unknown. There are instances in which correcting a laboratory abnormality is unhelpful or even causes increased mortality.[2,3,4,5,6] Even seemingly appropriate responses, such as returning the glucose or hemoglobin to normal, can have unpredictable effects.[5,6] Our understanding of the meaning of laboratory values in the ICU and how we should interpret and act on them is incomplete
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