To heparinize or not to heparinize: Effect on arterial blood gas measurements

Abstract

The most useful of investigations should be rapid and easy to perform, give reliable results and influence management decisions. The arterial blood gas goes a long way toward fulfilling the above needs. The utility of the arterial blood gas has been enhanced by the additional information provided by modern analyzers, including the plasma electrolytes, anion gap, ionized calcium and plasma lactate.[1] Its continuing reliability is dependent upon recognition of changes in the normal values with changes in the methods and technology employed for measurement. Samples not analyzed at the point of care require anticoagulation albeit in very small quantities, viz heparin 1 U/ml. The ideal anticoagulant would be dry, free of interference in laboratory tests, inexpensive and completely reliable as an anticoagulant.[2] The International Federation of Clinical Chemistry recommend for blood gas sampling, filling up of the dead space of the syringe with heparin, to lubricate the inner wall of the syringe, to expel the excess anticoagulant and to collect at least 20 times the dead space volume of blood.[3] In actual fact preanalytical sample handling is estimated to account for 75% of the errors in blood gas analysis.[4] In the study by Chhapola et al.[5] in this issue of the Journal examines an important aspect of arterial blood gas measurement, namely the influence on the various parameters of the type of heparin, dry balanced vs. liquid in the preparation of the sample. Dry balanced heparin is “electrolyte balanced,” (containing Lithium and Zinc rather than sodium or calcium) to prevent interference with the numerous electrolytes and other parameters estimated.[4] Heparin has two different effects on blood gas samples based on its intrinsic chemical properties and a dilution of the sample. As heparin dilutes mainly the plasma phase of the blood sample the magnitude of the dilution of a 1 ml blood sample by 0.05 ml of liquid heparin may be around 10%.[4] The authors have painstakingly prepared liquid heparin syringes with meticulous attention to detail regarding the pH, Na + and pO2 content of the heparin available in an attempt to standardize the technique. Further strengths of the study include a single needle prick for sampling with both syringes that minimized sampling differences. duplicate analysis of each sample giving a coefficient of variation of 1%, point of care analysis and achieving a sample size larger than calculated for pCO2. The authors however as stated did not measure ionized calcium a parameter previously shown to be influenced by various preparations of heparin.[2]