On the impact of seasonal variation in potassium results in primary care

Although we do not know the reason for the difference between potassium result distributions in ED and primary care, we do know that there are seasonal differences in primary care results that can be removed by specimen stabilisation. We can see the impact of this by looking at the predicted number of patients in primary care who would be placed in different categories if all practices performed at the level of Black Torrington and Lynton (Table 8)

	Black Torrington / Lynton	All GP
Low	0.015	0.013
Normal	0.98	0.96
Low High	0.0058	0.022
Moderate high	0.00075	0.0017
High high	0.000068	0.00027

Table 8  Actual proportion of potassium tests in different result categories comparing the two practices that stabilise all specimens by centrifugation, and all other practices 

From this analysis, we can work out the number of patients in primary care who would be placed in different categories if all practices stabilised specimens (Table 9).

Predicted number of additional patients in primary care who would have a low potassium if specimens stabilised

Low

168

	Predicted number of additional patients per year in primary care who have a high potassium due to seasonal variation
Low high	1953
Mod high	116
High high	25

Table 9 Predicted annual number of patients who fall into different result categories in primary care due to seasonal variation compared with practices that stabilise all specimens (based on 124,000 tests per year in primary care).

Conclusions

From the data in this blog we can estimate that about 2% of primary care potassium results across the year are mis-categorised as a result of specimen stability issues. If we combine this effect with the systematic shift in results described in the previous blog, we can estimate that around 1 in 10 potassium results from primary care would be significantly different if taken in the emergency department.

The current focus of laboratory accreditation has done little or nothing to address the issues that lead to this, and as a result patients and clinicians are being falsely reassured about the accuracy and validity of results. We can say with confidence that the current standard of practice is leading to patient harm on a large scale. This should be an issue of national importance.