GPs frequently find high serum potassium results unexpected, difficult to explain and therefore hard to interpret. The need for serum potassium to be urgently repeated causes anxiety and frustration to both patient and GP. Usually no explanation for the spuriously high serum potassium concentration (pseudohyperkalaemia) is found, but our informal local survey suggested around 50% of GPs thought a laboratory error was the cause.
Our study1 showed that pseudohyperkalaemia is almost invariably associated with factors affecting the pre-analytical stage. This includes the phlebotomy and transport temperature of blood samples to the laboratory. We carried out an audit of all GP requested potassium results over a 4-year period (224 000 samples). We confirmed the findings of other workers2 that there is an inverse relationship between mean serum potassium concentration for the primary care population and outdoor ambient temperature. The proportion of results above the normal range (5.2 mmol/L or higher) varied between 6% in hot weather to 17% in the coolest. This is partly due to passive movement of potassium from the intracellular to extracellular fluid as the energy dependant sodium/potassium pump activity declines as temperature falls. We reduced this frequency to between 4.5% and 9% (depending on external ambient temperature) by ensuring that phlebotomists did not facilitate venesection by asking patients to hand grip/fist clench. Potassium is released from myocytes into extracellular fluid during muscle contraction.
The majority of our ambulant GP patients attend hospital clinics for phlebotomy, all within 5 miles of the laboratory. Inter-site transport ensures that most samples arrive within 1 hour. However, proposed centralisation of pathology services (with laboratory closures) will increase sample transport time and distance, exacerbating the low temperature effect (unless heated transit vehicles are used). Provision of the phlebotomy service and appropriate sample transport may become an additional responsibility for those GPs who presently have these services provided by the hospital. Figure 1 shows the effect of outdoor ambient temperature and improving phlebotomy technique on the percentage of samples giving significant hyperkalaemia (5.8 mmol/L or higher).
Figure 1 Effect of temperature and phlebotomy on the incidence of hyperkalaemia.
Changes to pathology services may be introduced insidiously and GPs need to be aware of proposals that will affect their practice. Automated sample analysis can be performed in bulk on large analysers, but phlebotomy and pre-analytical handling require skill and knowledge. If this is overlooked in planned changes, news of pathology modernisation may be heralded by an epidemic of pseudohyperkalaemia. Periodic assessments of the incidence of hyperkalaemia in GPs' own practices can yield powerful information. If the incidence of moderate hyperkalaemia (5.8 mmol/L or higher) rises above 0.7% or >9% are above reference range, transport and phlebotomy arrangements should be reviewed.
- © British Journal of General Practice, 2008.
