Non specificity in diagnostic assays (interferences) is a problem. For example, using the search tool in Clinical Chemistry from 2006 to date, yielded 45 references. See reference 1 for one of the 45 (1).
Interferences can be thought of in two ways:
- a particularly bad interference will cause a huge error in a result
- a result can also exhibit smaller errors either by a single interference or by a combination of interferences, whose net effect is a smaller error.
Manufacturers study extensive lists of candidate interfering substances. Unfortunately, the way that many manufacturers report the results of interfering studies in package inserts is misleading with respect to case 2 above.
Manufacturers often cite a CLSI document – EP7A2 – which states that a claim can be that the following compounds: “were found not to interfere at the concentrations indicated.” But this only means that a bias of less than 10% was found. Later in the document, alternative bias claims are presented, which added to the first claim are:
- Substance did not interfere (< 10%)
- The observed amount of bias due to interference
- the maximum amount of bias due to interference that could occur
It’s not surprising that manufacturers tend to choose method 1 for claims. Besides being misleading, claim 1 is wrong statistically, since it amounts to stating that the null hypothesis has been proved, which is impossible.
There is an opportunity for CLSI to revisit its canceled standard – EP11 – Uniformity of Claims
- Dimeskia G, Jones B and Ungerer JPJ Interference from Rose Bengal with Total Bilirubin Measurement. Clin Chem 2009 55: 1040-1041.