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BACKGROUND AND OBJECTIVE. Rapid measurement of potassium is crucial in the resuscitation of patients in cardiac arrest. This is often done on an arterial blood gas sample taken during resuscitation and analysed in an emergency department based blood gas analyser. No-one has assessed how accurate or reliable this is when compared with the traditional method of sending a venous sample to the laboratory for standard analysis. This study looked at the agreement between potassium measurements in arterial blood gas samples and venous blood samples in patients in cardiac arrest. Method: Arterial and venous blood samples were taken at the same time and analysed in the usual way from 50 patients in cardiac arrest. It was found that the mean difference between each pair of arterial and venous potassium measurements was low at 0.106 mmol/l. However, the standard deviation of these dif- ferences and subsequently the 95% limits of agreement were wide (21.182 mmol/l to 1.394 mmol/l)— that is, 95% of differences will lie between these limits. It is felt that these limits are too wide for safe use in clinical practice. Conclusion: Based on these results, it is advised that arterial blood gas analysers should be used with caution to measure potassium in patients in cardiac arrest. A bnormalities of potassium concentration are consid- ered reversible causes of cardiac arrest.1 Hyperkalaemia causes hyperpolarisation of the heart, bradycardia, and asystole. Hypokalaemia, especially in the setting of heart disease and digoxin toxicity, causes fatal cardiac arrhythmias. In addition, the potassium concentration in blood frequently changes during the course of a resuscitation. Just after cardiac arrest the concentration rises due to the release of potassium from underperfused cells. Then, as resuscitation gets underway, there is often a rebound fall as the circulation is restored and endogenous adrenaline and exogenous drugs take effect.2 It is, therefore, important to measure potassium as soon as possible during the course of a resuscitation. This has traditionally been done by taking a venous sample and sending it to the biochemistry laboratory for standard analysis. This, however, can take some time. Many people now prefer to measure potassium on an arterial blood gas sample in a resuscitation room based blood gas analyser. This gives a much faster result, but it is not known how accurate this is when compared with a venous measurement. In this study we aimed to determine the agreement between potassium measurements in venous and arterial blood gas samples in patients in cardiac arrest. MATERIALS AND METHODS The emergency department records of patients who presented in cardiac arrest over a 10 month period in 2001 were traced and examined. A sample size of 50 was considered appro- priate as this is the minimum sample size recommended for a method comparison study.3 Patients were selected on the basis of both their arterial blood gas and venous laboratory blood results being available. This involved searching through emergency department, medical, and laboratory records until such records for 50 patients were obtained. At our hospital during resuscitation the arterial and venous blood samples are collected from either the radial or femoral artery and a vein. The arterial sample is taken directly into a preheparinised syringe and placed immediately in the arterial blood gas analyser in the ▇▇▇▇▇▇▇▇▇▇▇▇▇ ▇▇▇▇ (▇▇▇▇▇▇ Blood Gas Analyser). It takes approximately two minutes for the results to be ready. The venous sample is collected and sent to a central laboratory in the hospital, via a tube transport system, for analysis (Olympus Analyser). It takes approxi- mately 40 minutes for the results to be uploaded onto the hospital computer system or they can be obtained slightly more quickly via telephone. For the present study both sets of results were checked to make sure the dates and times of collection matched for each patient. For venous samples the time recorded was the time at which the blood samples were received by the laboratory as an emergency sample. For arterial samples the time taken was the time at which the samples were analysed by the arterial blood gas analyser. These times were checked to ensure that there was never more than 10 minutes between them and the time at which the patient was recorded as entering the resuscitation room. These three times were recorded automatically by the laboratory and by the arterial blood gas analyser, and in the nursing notes, respectively. The measurements for each patient were entered in a database for statistical analysis using Microsoft Excel for Windows (version 4.0, 1985–1997 Microsoft Corp, USA) and compared using methods described by ▇▇▇▇▇▇ for assessing the agreement between two methods of clinical measure- ment.3 RESULTS Both potassium measurements for each patient are given in table 1. The arterial potassium measurements ranged between