High ambient temperatures and hypokalaemia

We read with interest recent postings on the ACB mailbase relating to seasonal pseudohypokalaemia. ¹ At this time, we had noted an apparent increase in the incidence of hypokalaemia in samples from general practices (GP) analysed at University Hospital of Wales (UHW). Internal quality control monitoring revealed no shift in mean values on the analysers. The monitoring of daily patient means for all samples showed a decrease in mean potassium values for July 2013 and this prompted further investigation.

A retrospective data gather was performed for 10 working days 8–19 July 2013 inclusive, a period of prolonged higher than average daily temperatures. ² Data were also requested for 10 working days 9–20 July 2012 inclusive, during which temperatures were similar or cooler than average. ² Only samples from GP and UHW patients were included. Samples from outpatients were not differentiated from inpatients as there should be minimal delays in sample receipt in the laboratory. Statistical analyses were performed using IBM SPSS Statistics.

A significant reduction in the mean potassium concentration for both GP and hospital samples was observed in 2013 in comparison to 2012 (P < 0.001, Student’s t-test); however, the magnitude of the change was greater in GP patients (Table 1). An increase in the incidence of hypokalaemia (<3.5 mmol/L) was also observed in GP requests from 2012 to 2013; however, little difference was observed in hospital samples (Table 1). During the period of interest, eight patients with hypokalaemia on a GP sample had a repeat on a hospital sample within 48 h. Potassium concentrations were significantly higher on the hospital sample (P = 0.002, paired t-test) and five of eight hospital samples were within the reference range (3.5–5.3 mmol/L).

Table 1.

Mean potassium and incidence of hypokalaemia in comparative 10 working-day periods in July 2012 and July 2013.

GP 2012	GP 2013	Hospital 2012	Hospital 2013
Number of results	3678	4897	8222	8032
Mean potassium concentration (range), mmol/L	4.4 (1.5–6.4)	4.1* (1.7–8.0)	4.3 (2.2–8.6)	4.2* (2.1–8.6)
Number of samples potassium <3.5 mmol/L	51	325	495	558
Incidence of hypokalaemia (%)	1.4	6.6	6.0	6.9

Hospital includes samples from UHW inpatient and outpatients. Hypokalaemia defined as <3.5 mmol/L, according to the UHW reference range. *P < 0.001, 2013 vs. 2012 (Student’s t-test).

These findings confirm an increased frequency of hypokalaemia in GP patients during July 2013, consistent with a contribution from prolonged higher than average ambient temperatures. The phenomenon of seasonal pseudohypokalaemia has been reported previously in GP samples during the summer months^3–5 and appears to be related to increased Na⁺-K⁺-ATPase activity. ³ Interestingly, we also observed a shift in mean potassium of a smaller magnitude in hospital patients, similar to a previous report. ⁴ The effects of pseudohypokalaemia are likely to include time, cost, patient inconvenience and worry. ³ Seasonal pseudohypokalaemia should therefore be considered as a potential cause for unexplained hypokalaemia during hot weather. The possibility of pseudonormokalaemia masking hyperkalaemia is difficult to quantify but may have been significant during this period.