EP27 is a CLSI standard about constructing error grids for diagnostic assays and also about evaluating data in error grids. It was first published as a P (proposed) standard about a year ago. The A (approved) version EP27A has been stuck for a year!
So here is the objection to EP27A.
EP27 needs to define the various regions that form the error grid. The innermost region (zone A) is currently defined as the region which is thought to have a low potential for patient error and to include a high percentage of the data (often 95% or greater).
The commentator objects to having the percentage of data as part of the definition. The rationale is the definition should be based on clinical grounds, not on assay performance (e.g., percentages).
This objection seems reasonable but here’s why I favor the current definition.
“Clinical grounds” are stated as if they are limits set in stone. They are usually provided by asking clinicians, which is reasonable. As assay error increases, the probability of a clinician making an incorrect medical decision increases. Is there an amount of error which demarcates no patient harm from patient harm? I would submit the answer is no and that clinicians when asked to provide this limit, will give different answers, based on anecdotal data (as opposed to a clinical trial).
If a clinical trial could be performed, it would show the percentage of patients that are not harmed for cases where the limit has just been met. Say this is 90%. Now a bunch of other clinicians might come up with somewhat more stringent limit which might yield a percentage of unharmed patients as 95%. In this hypothetical trial, there are different patients with a variety of clinical symptoms and histories and different clinicians but the errors are all the same and at a specified limit. The point is that whatever limit is chosen (here the limit that results in 95% of patients with no harm is chosen) the limit will provide a percentage of patients which are unharmed and hence percentages are involved – call this percentage 1.
So the “clinical limits” imply percentages before any data is collected. The standard talks about how much data is in zone A – call this percentage 2 – not the percentage of patients that are unharmed. These two percentages are different.
Percentage 1 is not measureable although its intent is to be high (most patients unharmed). But percentage 1 will remain high only if percentage 2 is high. Thus, in the hypothetical trial if everything were the same then by definition, 100% of the errors would be just below the limit and 95% of the patients would be unharmed. Since 100% is not a practical number to specify, a high percentage will lead to close to 95% of the patients unharmed. Now if the errors are real, then the number of patients with no harm is unknown, but as long a high (95% or more) percentage of errors is within the limit, then the percentgae of unharmed patients will be close to 95%. Hence percentages should be part of the definition.
As a footnote, harm associated with exceeding zone A limits is considered as minor injury, as opposed to results in the LER zone (zone C) where LER = Limits of Erroneous Results and harm is associated with major injury or death.