Heparin anticoagulation of samples for blood gas analysis

Since the inception of blood gas analysis, heparin has been the anticoagulant of choice for preparation of samples. Historically, syringes used to collect arterial blood for gas analysis were prepared ”in house” by aspirating a small volume of liquid heparin (LH) and then expelling it. The thin film of liquid heparin that remains coated to the walls of syringe is sufficient to anticoagulate the blood sample. 

Nowadays, commercially prepared syringes containing preweighed dried ”electrolyte-balanced” heparin (DBH) are available. This newer method of preparing blood gas samples is recommended and widely used in the well-resourced hospitals of the developed world; but the older, cheaper LH method continues to be used in less well-resourced parts of the world. 

A recently published study conducted at the pediatric intensive care unit of an Indian hospital, where the LH method continues to be used, sought to establish if blood gas results derived from pediatric samples prepared using LH are comparable (i.e. without clinically significant discrepancy) to those obtained from samples prepared using the recommended DBH method.

Two simultaneously collected venous blood samples were obtained from 35 children admitted to intensive care; in all 35 cases, clinical condition indicated the need for blood gas analysis. The first of the two samples was collected into an ”in-house” prepared LH syringe, and the second was collected into a commercial DBH syringe. 

Both samples were treated in exactly the same way and submitted for blood gas analysis at the same time, using the same point-of-care blood gas analyzer. The eight parameters measured by the analyzer were: pH, pCO₂, bicarbonate, pO₂, potassium, sodium, chloride and lactate. 

Statistical analysis of paired results revealed that compared with samples prepared using the reference DBH method, there was a slight negative mean bias for all eight parameters for samples prepared using the LH method. This reached statistical significance (P <0.05) for all parameters except pCO₂ and potassium. 

Bland-Altman plots revealed that parameters with closest agreement were pH and lactate. For these two parameters at least, results of the study indicate that it is immaterial which of the two methods (LH or DBH) are used to prepare samples. However, there were clinically unacceptable levels of disagreement (expressed as exceeding total allowable error) for all other parameters. 

Sodium measurement showed an exceptional degree of disparity; of the 35 paired sodium results, 27 (77 %) exceeded total allowable error and were therefore clinically significantly different.

In summary this study has demonstrated that for most of the blood gas parameters assessed (pCO₂, pO₂, bicarbonate, potassium, sodium and chloride) there is clinically significant difference between samples collected using the LH method and samples collected using the DBH method. If pH and/or lactate are the parameters of interest then it does not matter which sample preparation method is used; results are apparently interchangeable.