An investigation of an unanticipated elevated potassium should include:
- Verification of sample identity; Check the tube labeling for correct patient name, medical record number, and date of birth.
- Examination of the serum aliquot tube or original collection tube for evidence of hemolysis.
- Examination of the collection tube for intact gel layer between cells and serum.
- Determination of a delay between collection time and time of receipt in the laboratory.
- Analysis of the calcium level to see if it is low or reduced from previous results.
The two primary sources of potassium in a blood sample are from red blood cells (RBCs) and from anti-coagulants, such as potassium EDTA. Once hemolysis is ruled out as a primary cause, delay in transport time and incomplete gel barrier in a serum separation tube may explain unanticipated levels of potassium. Finally, examining the calcium level may provide a clue as to the contamination of the sample with potassium EDTA, as potassium will be elevated, and calcium will be decreased due to the chelation binding with EDTA.
In this case, the cause was incorrect order of draw. The phlebotomist collected the lavender/purple-top tube just prior to the serum separator tube, which was a short draw. The potassium EDTA in the lavender/purple-top tube not only raised the potassium, but decreased the calcium. The test was cancelled and a redraw requested. The phlebotomist was retrained as to the required order of draw.