On the systematic difference between ED and GP potassium results

Primary care potassium results show large seasonal variation due to problems with specimen stabilisation. This may mask other underlying reasons for variation. If we plot the distribution of potassium results from ED and primary care side by side we can see what appears to be a systematic difference between the two populations (Figure 6)

Figure 6 Histogram showing potassium results obtained from ED and primary care.

In North Devon, two practices (Black Torrington and Lynton) have centrifuged all specimens on site since 2014. This has resulted in much more stable potassium results. We can see the same shift in mean potassium result in these practices that occurred when the analyser was changed in 2018. However, we also see a large systematic shift in the result between the primary care and ED populations (Figure 7).

Figure 7 Mean monthly potassium in ED compared with 2 practices that stabilise all specimens at point of care by centrifugation. Note that in this analysis we only looked at “first time tests” by removing any test that was repeated on a specific patient within 3 months

Impact of demographic differences on potassium result

One explanation for this result is that the people tested in ED are very different to those tested in primary care. The following population pyramid shows the differences in the demographic groups that are tested in ED compared with Lynton and Black Torrington (Figure 8).

Figure 8 Population pyramids showing demographic differences in people having a potassium test in the ED compared with Black Torrington / Lynton GP practices.

To examine whether these demographic differences could account for the systematic difference in potassium results, we looked at the results of first time testing (i.e. ignoring repeats within 3 months) in specific demographic groups. For simplicity, we looked at just the potassium results before the analyser change, as this gave us the largest dataset. We can see that the difference in mean potassium exists across all demographic groups (Table 4).

Table 4 Difference in mean potassium between ED and primary care in different demographic groups.

Impact of disease severity on potassium result

Patients in the ED who have a test are more likely to be acutely unwell than patients in general practice. In an attempt to determine whether the difference in potassium might be accounted for by differences in clinical reason for the test, we repeated our analysis looking at different groups that we could identify either through the clinical details on the request form, or from their testing history. The comparator group might be expected to contain patients who were more likely to be acutely unwell (Table 5). We can see no significant difference between the patient groups and their comparators, and conclude that disease severity is unlikely to be a reason for the systematic difference in potassium between the ED and primary care.

Table 5 Difference in mean potassium between patients in different clinical groups, as identified from clinical details on request form.

Location of patient group	Patient group	Comparator	No of patients in group	Number of patients in comparator	Mean K in group	Mean K in comparator	Difference
GP	Blood requested for chronic disease monitoring (asterisk in clinical details)	Not requested for chronic disease monitoring	667	3978	4.68	4.59	-0.09
GP	Patient has no repeat test within next week	Patient has repeat test in hospital within next week	162161	1381	4.46	4.51	0.05
ED	Patient has no repeat test within next week	Patient has repeat test within hospital within the next week	13996	1017	4.21	4.24	0.03
ED	Patient has no repeat test within next week	Patient has repeat test on ICU within the next week	13996	241	4.21	4.22	0.01

What does this mean for patients?

We cannot account for this systematic difference in potassium results between the ED and primary care. It does not appear to be due to demographic or disease differences. It would seem to be unlikely to be due to specimen stability as we see the effect in locations where specimens are stabilised at the point of draw and where results appear stable across the year. Both ED and primary care use the same equipment manufacturers and specimens are run on the same analysers.

Nonetheless, we can estimate the impact of this difference on the rate of abnormal results. The following table shows the proportions of patients who fall into different potassium result groups (Table 6).

Table 6 Proportion of potassium tests in different result categories comparing ED and the two practices that stabilise all specimens by centrifugation

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 and if analyser performance was the same as that seen for ED specimens (Table 7).

Table 7 Predicted annual number of patients who fall into different result categories using actual primary care mean compared with theoretical mean derived from ED results (based on 124,000 tests per year in primary care).

Conclusions

We will talk in a subsequent blog about the impact of specimen stability on potassium results. This blog looks only at the clinical impact of a systematic shift in result depending on the location of where the blood has been taken.. We can estimate that about 1 in 12 potassium results taken in primary care would be in a different result category if the test was done in a different location. We do not understand the reasons for this but clearly it is important to study this further, and to understand whether this is a phenomenon seen in other regions.