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.