Unusual Potassium result
The laboratory I work in has recently seen a very unusual potassium result. During the afternoon, one of our consultants came across a high potassium result, 13.4, that he suspected might be an EDTA contaminated sample.
A potassium of 13.4 mmol/L is grossly abnormal and incompatible with life. We were certain the result did not reflect what was happening within the patient but we were at a loss to explain the actual cause of the elevated potassium.
There are three common causes of falsely elevated potassium.
- Delay in centrifugation
- heamolysis
- EDTA contamination
Centrifugation allows the serum or plasma to be seperated from the cells within blood. Some sample tubes contain a gel material that actually forms a physical barrier during the centrifugation process. Delaying centrifugation means that the serum or plasma is in contact with the cells for a greater length of time.
Once the blood sample is collected, the cells within the sample only have a finite amount of resources to use. When the cells were freely circulating around the body, they had easy access to resources like glucose and oxygen. As time goes by, some of the cells can’t cope with the conditions and start to die. This process results in the cells lysing releases their contents or become leaky allowing some of their contents to leave e.g. potassium. Lysis of red blood cells releases large concentrations of lactate dehydrogenase, phosphate and potassium. This is why a delay in centrifugation causes an increase potassium, it’s due to the red blood cells releasing potassium.
The date on the sample indicated that there was no significant delay in centrifugation. This suggested another cause of the elevation. The sample was not heamolysed so EDTA contamination was considered as the likely cause. A calcium was added to the sample and put onto the analyser. The result for the calcium was 2.4 mmol/L. Thus rulling EDTA contaimination out.
The results did not fit the picture. A potassium of 13 mmol/L is incompatible with life so we can rule out the result as being genuine. Another possible cause of problems is contamination with a tube for glucose collection. These sample contain a source of potassium. This was ruled out as the patient didn’t have a glucose sample collected and the potassium result was not high enough.
Our consultant investigated the patient further and contacted the person who collected the sample. The sample was collected the day before with the intention of dropping the sample into the laboratory on their way home. When the individual got home and found the sample, the sample was placed in their fridge overnight. This increased the amount of potassium released from the red blood cells. When the sample was taken out from the fridge the next day, the individual noticed they had forgotten to write the date on the form and sample and wrote todays date, not the date of collection. Mystery solved!
An unusual source of elevated potassium is due to the clotting process. In some individuals, the clotting process can cause potassium to be released giving the impression of an elevated potassium concentration. One way of determining this is to collect serum and plasma samples for analysis. The serum sample should have a significantly higher potassium concentration compared to the plasma sample because of the clotting process. Plasma samples are prevented from the clotting by the use of heparin in the sample.