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Guidance 18: Impurities in drug substances and drug products

9 August 2013

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18.4 Measuring impurities - rounding

In many pharmacopoeial tests for impurities, compliance (or otherwise) with the test is determined by direct comparison of high-performance liquid chromatography (HPLC) peak areas of impurities with those of standards.

The limit is the peak area of the standard; if a numerical limit is given in parentheses, it is only an approximate limit, for information.

Where a numerical limit is specified as the primary decision criterion, the calculated result of the test is first rounded to the number of significant figures given in the limit before the decision criterion is applied.

If the number of decimal places is greater than the number specified in the limits, the data should be rounded in accordance with the BP. If the last figure is 0–4, it is rounded down; if the last figure is 5–9, it is rounded up.

For example: A result of 0.14 per cent complies with a specified limit of not more than 0.1 per cent, but would not comply with a limit of 0.10 per cent.


  • An HPLC limit for an identified impurity, expressed as a percentage, generally means percentage by weight (% w/w) relative to the nominal content of drug substance, unless otherwise specified.
  • For an unidentified impurity, it generally means percentage by area (% a/a).

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