Relative risks of different types of contact lenses.

potassium concentrations by flame photometry (within batch SD=0 03 mmol/l). The Mann-Whitney U test and Spearman's rank correlation coefficient were used in statistical analysis. The median (range) difference in potassium concentration between serum and plasma in the 43 patients with rheumatoid arthritis was 0 3 (0 1-1 3) mmol/l, which was significantly greater (p<0 001) than that in the 21 controls with arthritis (0 2 (0-0 4) mmol/l) and the 28 normal subjects (0 2 (0-0-4) mmol/l). Serum creatinine and plasma potassium concentrations and the white cell count were similar in the three groups, but the platelet count was significantly (p<0-001) higher in the patients with rheumatoid arthritis (median (range) 366 (165-1260)x 103/1 v 287 (122-444)x 107/1 in the controls with arthritis and 305 (192-440)x 107/1 in the normal subjects). The difference in potassium concentration between serum and plasma in the patients with rheumatoid arthritis correlated with their platelet count (figure) (r=0 64, p<0001). Comment We are not aware of other reports of pseudohyper-kalaemia occurring in conditions such as rheumatoid arthritis. We found that in normal subjects the median serum potassium concentration was 0-2 mmol/l higher than the plasma potassium concentration, which confirms a previous report.4 We found a similar difference in our control group of patients with arthritis who were taking non-steroidal anti-inflammatory drugs, which excludes these as the cause of the pseudohyperkalaemia. In contrast, the difference in concentrations was significant in the patients with rheumatoid arthritis, with values of 1 5 mmol/l being recorded (case 2). The pseudo-hyperkalaemia may have been due to the raised platelet count in view of the correlation between it and the difference between serum and plasma potassium concentrations. Failure to recognise pseudohyperkalaemia in our patients led to errors in management: one patient was given treatment to lower her potassium concentration