Background Current sampling and transport conditions of samples in general practice can result in pseudohyperkalaemia. This study was undertaken to determine, in a general practice setting, whether there is any difference in haemolysis obtained when using BD Vacutainer® Rapid Serum Tubes (BD RST) compared with using BD Vacutainer® SST™ II Advance Blood Collection Tubes (BD SSTII). Methods Blood was collected from 353 patients requiring blood sampling who were attending 31 general practitioner practices in Belgium. For each patient, two BD SSTII tubes and two BD RST tubes were drawn in a randomized order. One of each pair of tubes was inverted five times, the other was not. Serum potassium concentration, serum LDH activity and haemolysis index were measured in each sample. Results There was no significant difference in measured potassium concentration according to tube type ( P = 0.16). Measured LDH activities were 1.7% higher in serum collected into BD SSTII tubes compared to BD RST tubes ( P = 0.02). When comparing serum from unmixed BD RST with BD SSTII tubes, there was a slight reduction in the haemolysis index but no significant difference in measured potassium concentration or LDH activity. Risk of hyperkalaemia was 4.8 times higher in serum from tubes that were incompletely filled compared to those that were filled with the correct amount of blood. Conclusion Both types of blood tubes are suitable for the measurement of serum potassium and LDH in patients from general practice. Tube inversion does not improve the accuracy of either serum potassium or LDH measurement. Blood tubes should be filled to the level recommended by the manufacturer to avoid artefactual increases in measured serum potassium concentration and LDH activity.