Pseudohyperkalemia during the onset of critical illness

In everyday practice we observe a difference between the potassium levels measured by routine biochemistry and by the gas analysis machine. This is expected since the former measures serum potassium while the later measures plasma potassium. The serum/plasma potassium concentration difference is related to the patient's platelet count and increases to statistically significant levels in thrombocytaemia. We measured the serum/plasma potassium difference of ICU patient within the first 6 hours of their admission and compared it with the difference measured in chronically severely ill (uremic) patients, as well as a group of healthy volunteers.

We measured the serum potassium (SKA), plasma potassium (PKA), serum/plasma potassium difference (SPdif-A = SKA – PKA), age and platelet (PLT) count in 50 ICU patients (Group A). They had been previously healthy and had suffered a major catastrophe (road traffic accident, gastrointestinal bleed, major trauma) less than 6 hours from the measurements. We also measured the serum potassium (SKB), plasma potassium (PKB), serum/plasma potassium difference (SPdif-B = SKB – PKB), age and PLT count in 31 chronic uremic patients on renal replacement therapy (Group B). The patients had come for treatment as scheduled, with no evidence of any other acute disease.

The same variables (SKC, PKC, SPdif-C) were measured in 20 healthy volunteers (Group C). Blood samples were obtained from the radial artery. Serum potassium was collected in a Vacutainer tube, Plasma potassium in a syringe treated with heparin, and platelet counts were measured in EDTA-treated plasma samples. Potassium level were measured using the same equipment.

Results

Group A: male/female = 37/13, age (years) = 63 (19), PLT × 1000/μl = 259 (83), SKA (mmol/l) = 4.31 (0.57), PKA (mmol/l) = 3.69 (0.52), SPdif-A (mmol/l) = 0.62 (0.23). Group B: male/female = 25/6, age = 59 (15), PLT = 235 (66), SKB = 4.89 (0.85), PKB = 4.55 (0.81), SPdif-B = 0.34 (0.16). Group C: male/female = 10/10, age = 49 (11), PLT = 220 (62), SKC = 4.14 (0.52), PKC = 3.90 (0.48), SPdif-C = 0.24 (0.12). There was a significant difference of SPdif between Group A/Group B, Group A/Group C and Group B/Group C (P < 0.001).

Conclusions

We suggest that plasma potassium is more reliable than serum potassium in the critically ill, since the latter depends on the platelet count. We should evaluate the patient and act according to the plasma levels, since pseudohyperkalemia is more likely, even for normal platelet counts. The higher Spdif observed in the critically ill implies that a number of their platelets is not counted with conventional methods, probably because they are activated and aggregate during the initiation of the acute phase reaction.