Impact of patient characteristics on posture-induced hematologic and biochemical variations in critical care patients

Laboratory reference ranges and phlebotomy guidelines are derived from ambulatory individuals, whose values reflect reduced plasma volume from hydrostatic pressure shifts. These conditions may not represent intensive care unit (ICU) patients, who remain supine for extended periods. This study evaluated the impact of posture on routine hematologic and biochemical laboratory tests in ICU patients.

A prospective, single-center study with a within-subject design enrolled adult, hemodynamically stable ICU patients. Blood samples were collected first in supine patients and again after 15 minutes in upright patients. Paired specimens were compared for differences in routine hematologic and biochemical test values.

Forty-four patients, with a mean (SD) age of 65 (14) years, were included. Statistically significant posture-related differences were observed in red and white blood cell counts, hemoglobin, hematocrit, potassium, carbon dioxide, anion gap, albumin, alkaline phosphatase, and total bilirubin (P ≤ .05). Correlation analyses showed associations between atherosclerosis and white blood cell count, antibiotic use, and hematocrit; liver disease or sepsis was associated with red blood cell distribution width. Potassium levels were correlated with cardiovascular disease, glucose with trauma, and total protein with intracranial hemorrhage.

Upright positioning for 15 minutes improves the accuracy of routine laboratory testing in ICU patients and should be considered in phlebotomy protocols.