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Published: August 8, 2022

Q. How do you calculate the swab limit for cleaning validation, once it has been extracted in solution?

A.  Swab limits are an expression of an acceptance criteria converted to an amount of residue detected in a swab.   Swab limits can be expressed as residue per swab, or as residue per mL in the extract sample. In general the swab limit for cleaning validations calculations go as follows:

  • swab limit (ug residue/swab) = acceptance criteria (ug residue/cm2) X swab area (cm2) X swab recovery (%);
  • acceptance criteria = ug residue/cm2 of a given residue;
  • swab area = cm2 of area swabbed
  • swab recovery = the percent actually detected compared to a spiked surface area of known amount of residue (amount detected/amount swabbed X 100).

To convert swab limits to test result limits by converting ug residue/swab to ug residue/mL based on the extraction volume that is used to extract the swab for analysis. The general equation is:

  • test result limit (ug residue/mL) = swab limit (ug residue/swab) / mL of extraction solution.

If the analytical method being used is total organic carbon, you can convert the test result limits to TOC concentration limits by using the known percent TOC content of the residue in question:

  • TOC concentration limit (ug TOC/mL) = test result limit (ug residue/mL) * %TOC(w/w) of Residue (ug TOC/100 ug Residue)

The same concept converting a test limit to a TOC limit can be used with any other analytical method that is using a marker to detect residue.  For example if HPLC is being used to detect a specific surfactant in a detergent you can covert a test limit to a surfactant concentration limit by using the known surfactant content of the detergent residue using a similar equation:

  • Surfactant Concentration (ug Surfactant/mL) = test limit (ur residue/mL)  * % surfactant in detergent (ug surfactant/100 ug detergent residue)

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