Correct Blood Sampling for Blood Gas Analysis

ABG analysis is an essential part of management of critically ll patients admitted in ICU/ICCU. The clinical biochemists have to face a very common complaint that ABG results from their laboratories are not satisfactory. But ABG measurements are particularly vulnerable to many pre-analytical errors. The most common sources of error encountered are improper concentration of anti-coagulant in the sample, air bubbles in the sample and delayed transport of non-cooled sample. For ABG, heparinized arterial blood sample is used. The correct heparin to blood ratio is very important to obtain accurate results and to prevent blood coagulation. According to International Federation of Clinical Chemistry (IFCC), final Heparin to blood ratio should be 50U heparin per ml arterial blood for accurate ABG results. So, syringe should be heparinized with 1000 U/ml of heparin vial, and then volume of blood equal to 20 times the dead space should be added. Dead space of a syringe varies according to the size of syringe and needle, in which liquid heparin remains even after complete flushing. Underfilling of syringe causes dilutional effects of heparin, which is the most common cause of error. Plasma electrolytes (particularly Sodium), pC[O.sub.2] and Glucose values decrease linearly with dilution of plasma. Hence this may result in false low values of these parameters. Commercial heparin is also available in 5000 U/ml and 30,000U/ml concentrations. So, using correct strength of heparin vial is equally important while preparing ABG syringe. Another very common error is due to air bubbles in ABG sample. A single air bubble may seriously affect p[O.sub.2] value. Last but not least is delayed transport of non-cooled sample, in which we get erroneous results due to cell metabolisms. It is concluded that for ABG samples, preanalytical errors must be minimized so that critically ill patients are given accurate quality reports.