Mathematical Correction of the Effects of Storage Time and Gas Contamination on Blood Sample Measurements

Bahareh Nevirian^1*Lars Pilegaard Thomsen¹Steen Kåre Fagerberg^2,3Jette Nybo^2,4Mette Krogh Pedersen³Kjeld Asbjørn Jensen Damgaard³Lisha Shastri¹Søren Risom Kristensen^2,4Stephen Edward Rees^1*

• ¹Aalborg University, Respiratory and Critical Care (Rcare) Group, Aalborg, Denmark
• ²Aalborg University, Department of Clinical Medicine, Aalborg, Denmark
• ³Aalborg University Hospital, Anesthesia and Intensive Care Unit, Aalborg, Denmark
• ⁴Aalborg University Hospital, Department of Clinical Biochemistry, Aalborg, Denmark

Objective: Analysis delay and gas contamination can affect the accuracy of blood measurements. This study uses a mathematical model of blood acid-base chemistry and gas in the sample tubes to calculate values of pH, pCO2, pO2, SO2, glucose, and lactate at sample time from measurements with delayed analysis and gas contamination. Methods: Data were analyzed from two published studies. Study 1: samples were obtained from thirty critically ill patients in standard blood gas syringes and analyzed after 0, 36, 54, 72, 90, 108, 126, 144, 162, and 180 minutes. Study 2: samples were taken from 20 healthy participants in standard blood gas syringes and vacuum tubes (2 mL and 4 mL) and analyzed after 0, 20, and 90 minutes. Calculated values from the mathematical model were compared to measured values at sample time. Result: For delay of up to 90 minutes, the accuracy (mean) and precision (SD) of values calculated at the sample time using syringes and 4 mL vacuum tubes remained within clinically acceptable limits when compared to measured values, with the exception of SO₂ in vacuum tubes. Values represent the mean difference ± standard deviation of the difference between calculated and measured values. For syringes pH=-0.004±0.011, pCO₂ =0.08±0.18 kPa, pO₂ =0.05±0.34 kPa, SO₂ =0.39±2.21 %, Glucose =0.07±0.35 mmol/L, Lactate =0.13±0.22 mmol/L, and for 4 mL vacuum tubes pH=0.006±0.007, pCO₂=-0.07±0.11 kPa, pO₂=-0.37±0.34 kPa, SO₂=-7.79±4.95 %, Glucose=0.01±0.11 mmol/L, Lactate=-0.00±0.20 mmol/L. 2 mL vacuum tubes had poorer accuracy and precision when compared to syringes and 4 mL vacuum tubes in a subset of cases. Conclusion: This study has shown that a mathematical model can calculate accurate and precise values at sample time following delayed analysis for both standard blood gas syringes and some vacuum tubes. This method may have clinical application in improving the logistics of blood sampling and analysis.