A limit test is a type of analytical method used to determine whether a specific impurity or substance in a pharmaceutical product is present at a concentration below a predefined acceptable limit.
Limit tests are typically employed when the impurity or substance is not expected to be present in significant amounts, or when the impurity is potentially harmful, and its concentration must be controlled to ensure the quality, safety, and efficacy of the drug product.
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Limit tests can be qualitative or quantitative:
1) Qualitative limit tests:
These tests provide a pass/fail result, indicating whether the impurity or substance is present below or above the specified limit.
Qualitative limit tests often involve the comparison of the test sample with a reference standard, such as comparing the color intensity or the size of a precipitate.
2) Quantitative limit tests:
These tests determine the actual concentration of the impurity or substance in the sample.
Quantitative limit tests typically involve analytical techniques such as high-performance liquid chromatography (HPLC), gas chromatography (GC), or atomic absorption spectroscopy (AAS), which can accurately measure the concentration of the impurity or substance.
Limit tests are commonly used for the following purposes:
1) Heavy metals:
To ensure that heavy metals, such as lead, cadmium, mercury, or arsenic, are present below their acceptable limits due to their potential toxicity.
2) Residual solvents:
To verify that solvents used during the synthesis or purification process are present below their acceptable limits, as some solvents can be toxic or have undesirable effects.
3) Preservatives and antioxidants:
To confirm that the concentrations of preservatives or antioxidants in a drug product are within the acceptable range to ensure product stability and prevent microbial contamination.
4) Microbial contaminants:
To assess whether the microbial load in a pharmaceutical product is within the acceptable limits to ensure product safety and efficacy.