Bottom Line Up Top: Blood gas electrolytes closely correlate with serum measurements and can be used under most circumstances to guide clinical care.
Clinical Scenario: A 62 year old woman with a history of HTN, ESRD on MWF dialysis presents to the Emergency Department with generalized weakness. Her vital signs are HR = 82, BP 153/72, O2 Sat 95%, Temp 98.7. She was unable to get dialysis today as her fistula was found to be non-functional. You are concerned about hyperkalemia and order labs, a venous blood gas and an ECG. The ECG is inconclusive without clear signs of hyperkalemia. The blood gas returns with a K = 5.4 and you call to admit the patient to get a fistula revision but the admitting team insists on waiting for the basic metabolic panel since blood gas electrolyte levels aren’t accurate.
What Your Gut Says: Blood gas electrolyte measurements are unreliable. You can wait on admitting the patient until the basic metabolic panel is resulted.
What The Evidence Says:
The concern for electrolyte discrepancy between blood gas and standard chemistry tests comes from concerns about the devices used for assessment. Point of care analyzers (blood gas machines) can rapidly return pH, lactate, hematocrit and electrolyte data giving clinicians incredibly useful information to guide management. Serum tests run by lab analyzers are the standard approach to obtaining these data points but can result in prolonged delays. If blood gas results are accurate, time to intervention and disposition can be reduced.
For a topic that gives clinicians a great deal of angst, there’s actually considerable data to answer the question. The major electrolytes investigated were sodium and potassium. An observational study of 200 paired arterial blood gas (ABG) and standard chemistry samples found no statistically significant difference for potassium levels and a statistically significant but clinically meaningless difference for sodium levels (131 mEq (ABG) vs 136 mEq) (Jain 2009). Similar results were found for 219 paired samples from ICU patients with statistically significant but clinically meaningless differences for sodium and potassium (Triplett 2016). Paired samples from over a thousand patients showed strong correlation between blood gas and standard samples with correlation coefficients of 0.82 for potassium and 0.85 for sodium (Uysal 2016). These researchers conclude that values from blood gasses can be used to guide clinical care though confirmation with a serum chemistry is advised. A 2022 study demonstrated no significant difference in potassium levels when comparing blood gas and standard samples across 265 patients with paired blood samples (Mahmoud 2022).
We have additional data of close correlation in specific disease states. A study of 165 patients with COPD found statistically significant differences in potassium and sodium levels between paired samples but, the differences were clinically insignificant (potassium mean difference of 0.19, sodium mean difference of 0.5) (Tang 2024). Paired samples in patients with DKA showed strong correlation between sodium and bicarbonate levels but a weaker correlation for chloride (Menchine 2011).
One caveat to keep in mind is that blood gas analyzers do not give information about hemolysis. As a result, an elevated potassium level on a blood gas should still be followed by an ECG looking for signs of hyperkalemia to guide management.
Bottom Line: Blood gas electrolytes are strongly correlated with serum chemistry results. The rapidity with which these results can be obtained makes them extremely valuable in guiding clinical care.
Read More
LITFL: https://litfl.com/vbg-versus-abg/
REBEL EM: https://rebelem.com/the-veinart-trial-vbg-vs-abg/
References
Jain A et al. Comparison of the point-of-care blood gas analyzer versus the laboratory auto-analyzer for the measurement of electrolytes. Int J Emerge Med 2009; 2: 117-20. PMID: 20157454
Menchine M et al. Diagnostic accuracy of venous blood gas electrolytes for identifying diabetic ketoacidosis in the Emergency Department. Acad Emerg Med 2011; 10: 1105-8. PMID: 21951652
Uysal E et al. How reliable are electrolytes and metabolite results measured by a blood gas analyzer in the ED? Am J Emerg Med 2016; 34(3): 419-24. PMID: 26658635
Mahmoud H et al. Accuracy of potassium measurement using blood gas analyzer. Cureus 2022; 14(3): e23653. PMID: 35371883
Tang S et al. Comparative analysis of hemoglobin, potassium, sodium and glucose in arterial blood gas and venous blood of patients with COPD. Nature 2024; 14: 5194. PMID: 38431760
Triplett KE et al. Can the blood gas analyser results be believed? A prospective multicentre study comparing haemoglobin, sodium and potassium measurements by blood gas analysers and laboratory auto-analysers. Anaesth Intensive Care 2019; 47(2): 120-7. PMID: 31070468
