Effect of water intake on routine laboratory test parameters in healthy adults: a cross-sectional study

Fasting before routine blood sampling has been regarded as a standard requirement. However, this practice may cause unnecessary discomfort and lacks sufficient scientific evidence. This study aimed to investigate the effect of water intake on commonly measured complete blood count (CBC) and biochemical parameters.

This two-step study involved two homogeneous cohorts of healthy adult medical postgraduate students. In Step 1, participants from Cohort 1 (n = 38) underwent standard fasting conditions to establish the short-term biological variation (BV) and corresponding reference change values (RCVs). In Step 2, participants from Cohort 2 (n = 33) had blood analysis before and one hour after consuming 500 mL water to assess the resulting changes, and only mean differences (MDs) exceeding the corresponding RCVs were considered clinically significant.

Twelve CBC parameters and 7 biochemical parameters showed statistically significant differences one hour after the intake of 500 mL of water compared with pre-intake values, including leukocytes, lymphocytes, monocytes, erythrocytes, hemoglobin, hematocrit, mean cell volume, mean cell hemoglobin, mean cell hemoglobin concentration, red cell distribution width-standard deviation, platelets, plateletcrit, alkaline phosphatase, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, urea, uric acid, and creatinine. The remaining 6 CBC parameters and 5 biochemical parameters showed no statistically significant differences, including neutrophils, eosinophils, basophils, mean platelet volume, platelet distribution width, platelet larger cell ratio, alanine aminotransferase, aspartate aminotransferase, γ-glutamyltransferase, triglyceride, and glucose. Notably, only the change of lymphocytes was considered clinically significant.

Water intake before routine blood sampling has minimal clinically relevant effects on commonly measured CBC and biochemical parameters in healthy adults.