Bioequivalence and Bioavailability Forum

Dear Satya!

❝ Specificity and Selectivity are the two related but confusing terms

❝ described in a bioanalytical guidelines.

Guidelines should talk about selectivity only.
According to IUPAC:
Specific is considered to be the ultimate of selective, meaning that no interferences are supposed to occur.

...which is a rather theoretical state...
Therefore in the real world we should assess selectivity; again according to IUPAC:
selective (in analysis)
A term which expresses qualitatively the extent to which other substances interfere with the determination of a substance according to a given procedure.

❝ To me specificity is a matrix related term and selectivity an analyte

❝ related term.

Selectivity depends on the analyte, metabolites, degradants, co-administered compounds, matrix components,...
But you are right, it's a lot of confusion out there...

Unfortunately FDA (2001) makes ambigous statements:

A. Selectivity
Selectivity is the ability of an analytical method to differentiate and quantify the analyte in the presence of other components in the sample. For selectivity, analyses of blank samples of the appropriate biological matrix (plasma, urine, or other matrix) should be obtained from at least six sources. Each blank sample should be tested for interference, and selectivity should be ensured at the lower limit of quantification (LLOQ).
Potential interfering substances in a biological matrix include endogenous matrix components, metabolites, decomposition products, and in the actual study, concomitant medication and other exogenous xenobiotics. If the method is intended to quantify more than one analyte, each analyte should be tested to ensure that there is no interference.

but in the same document at...

F. Specific Recommendations for Method Validation
The specificity of the assay methodology should be established using a minimum of six independent sources of the same matrix. For hyphenated mass spectrometry-based methods, however, testing six independent matrices for interference may not be important. In the case of LC-MS and LC-MS-MS-based procedures, matrix effects should be investigated to ensure that precision, selectivity, and sensitivity will not be compromised. Method selectivity should be evaluated during method development and throughout method validation and can continue throughout application of the method to actual study samples.

So the statement in section A is correct (talking about selectivity), whereas the one in section F (talking about specificity) is not. Actually I would guess it's just a typo. Or does the FDA want to contradict itself within the same document and the International Union of Pure and Applied Chemistry?

Specificity instead of selectivity is also used in other guidelines (EU, South Africa,...), whereas Argentina's ANMAT talks about selectivity only.

A reasonable definition is given by ANVISA:

2.1. Specificity and Selectivity
It is the capacity of the method to accurately measure a compound in the presence of other components such as impurities, degradation products and components of the matrix.
2.1.1. For qualitative analysis (identification test) it is necessary to demonstrate the capacity of selection of the method between compounds with related structures that may be present. This must be confirmed by obtaining positive results (preferably in relation to known reference material) in samples containing the drug, comparatively with negative results obtained with samples that do not contain the drug, but structurally similar compounds.
2.1.2. For quantitative analysis (content) and analysis of impurities, specificity can be determined by the comparison of the results obtained from samples (drug or drug product) contaminated with appropriate amounts of impurities or excipients and samples not contaminated, to demonstrate that the result of the test is not affected by these materials. In the absence of impurity or standard of the degradation product, the specificity of the method can be determined by comparing the results of analysis of samples containing impurities or degradation products with the results of a second, well characterized procedure (e.g. pharmacopeial methodology or other validated procedure). These comparisons must include samples stored under stressful conditions (e.g. light, heat, humidity, acid/base hydrolysis and oxidation).
2.1.3. In chromatographic methods, the necessary precautions must be taken to guarantee the purity of the chromatographic peaks. The use of peak purity tests (e.g. with the aid of photodiode arrangements or mass spectrometry) is interesting to demonstrate that the chromatographic peak is attributed to a single component.

For a nice glossary see Table 2 of:
S Bansal and A DeStefano
Key Elements of Bioanalytical Method Validation for Small Molecules
The AAPS Journal 9(1), E109-E114 (2007)
online PDF