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What are the probable reasons for the increase in assay during the stability study of a drug product

An increase in assay value during a stability study of a drug product can be indicative of various factors. It's important to understand that an increase in assay value implies a higher concentration of the active pharmaceutical ingredient (API) than expected. Here are some probable reasons for this observation:

1. Change in polymorphism:

Polymorphism in pharmaceuticals refers to the occurrence of a substance in more than one form or crystal structure. These different forms can exhibit significantly different physical and chemical properties, such as solubility, melting point, and chemical reactivity. Many drug assays, particularly those based on spectroscopic techniques like UV-Vis absorption, are sensitive to the physical and chemical environment of the molecule. Different polymorphs can exhibit different spectral characteristics, leading to variations in the absorption intensity at a specific wavelength. If a polymorph change leads to increased absorption, the assay might indicate a higher concentration of the drug than actually present.

Fig-1: Normalized absorption spectra of polymorph-α, polymorph-β and [Zn(phen) 3 ] 2 L(ClO 4 ) 4 in the solid state.

2. Degradation of Excipients or Impurities:

If excipients or impurities in the formulation degrade over time, they might interfere with the assay, leading to falsely elevated API levels. This is particularly relevant for methods like HPLC where peaks of degradants might overlap or interfere with the API peak.

3. Changes in the drug product’s pH:

The pH of a drug product can significantly influence its chemical and physical properties, impacting drug stability, solubility, and the performance of analytical methods used for assay determinations. The changes in pH can lead to higher assay results in drug product analysis. Many drugs exist in ionized or non-ionized forms, depending on the pH. The ionization state can affect the drug's interaction with the assay method, potentially leading to a higher measured concentration.

Changes in the pH of a drug product can significantly impact its analysis by UV spectroscopy, potentially leading to higher assay results. This is primarily due to the influence of pH on the chemical and physical properties of the drug and its interaction with the detection method. Many drugs exhibit pH-dependent absorption spectra in UV spectroscopy. The absorbance of a compound at a specific wavelength can vary depending on the pH of the solution, affecting the intensity of the UV light absorbed. If the pH change causes increased absorbance at the detection wavelength, the assay might indicate a higher concentration of the drug than is actually present.

Fig-2: pH-induced changes in UV–vis absorbance spectra of dipicolinic acid (DPA)

4. Concentration Changes Due to Evaporation:

If the formulation is volatile and stored in conditions where evaporation can occur (like improperly sealed containers), this might concentrate the API, thus increasing the assay value. This is possible for a aqueous-based drug product packaged in a semi-permeable container

5. Method Sensitivity and Specificity:

The analytical method might lack specificity, resulting in the detection of other components as the API. This is often the case with non-specific analytical techniques such as UV spectroscopy.

6. Chemical Interactions:

Sometimes, interactions between the API and excipients or container-closure systems can lead to chemical changes that increase the apparent concentration of the API.

7. Instability of the API:

In rare cases, degradation products of the API might reconvert to the API under specific conditions, artificially increasing its concentration.

8. Sampling Errors:

Non-uniform distribution of the API in the product and inconsistent sampling can lead to variability in assay results, including occasional increases.

9. Analytical Error:

Inaccuracies in the analytical method used for the assay can lead to erroneous results. This could be due to calibration errors, improper sample handling, or operator errors.

It's crucial to thoroughly investigate any unexpected increase in assay values during stability studies, as this can impact the quality, efficacy, and safety of the drug product. Often, a combination of analytical method review, additional stability testing under varied conditions, and thorough examination of the formulation and packaging components are necessary to identify the root cause


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Rated 4 out of 5 stars.

Movement of API could be the reason in case of cream and ointment at Acc.condition

Replying to

Agree pandey sir


Rated 5 out of 5 stars.



Rated 5 out of 5 stars.

Excellent article. Highly informative.


Rated 5 out of 5 stars.

Excellent article. Highly informative.


Apr 05

Excellent article. Highly informative. However, in addition to what the author said I am stressing the importance of quality of ref standard against that previous and current measurement are done.

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