Here’s something you may see sometimes: a patient presents with a (non-hemolyzed ) potassium of 7.0. Scary! But you check a potassium on VBG, and it returns 5.5. Which is the right value? What’s going on here?
Serum potassium, generally speaking, is more accurate. Serum samples contains potassium released by platelets (which are separated out in the phlebotomy tube) so is usually 0.1-0.7 mmol/L higher than plasma samples. Whole blood potassium is usually accurate enough, especially when you need a potassium level quickly in a critical care/emergency setting.
|Serum potassium||Whole blood (plasma) potassium|
|Type of tube||Serum-separating tube “tiger top,” “gold top,” “marble top”||Heparin whole blood tube “green top” for a VBG or ABG kit|
|Timeliness||Rapid turnaround time|
|Accuracy||More accurate||Usually accurate, but may underestimate hyperkalemia|
|Affected by hemolysis?||Yes||No|
In the example above, there is a significant difference between the serum and whole blood potassium, suggesting that there may be a degree of pseudohyperkalemia.
- Did the patient clench their fist or have a traumatic blood draw?
- Was the patient in acute respiratory alkalosis when the blood was drawn?
- Does the patient have thrombocytosis? (Platelets can release potassium, causing false elevation)
- Is the specimen hemolyzed–has it been sitting in the lab for a long time before being processed?