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The Analytical Evaluation Threshold (AET) is critical in developing your analytical approaches to identifying and quantifying extractables and leachables (E&L). This mathematical calculation is used to convert a dose-based threshold to a concentration-based threshold, which the analytical chemist then uses, to ensure that the analytical methods employed meet the required levels of sensitivity.

What is the AET Analytical Evaluation Threshold?

The AET concept was originally developed by the PQRI best practices group working on the ‘safety thresholds and best practices for E&L in orally inhaled and nasal drug products’ guideline (2006). 

It is important to stress that the AET is not a safety threshold but an identification threshold used by the analytical chemist, once an extractable or leachable has been identified and quantified a toxicologist can assess whether that particular species represents a risk to the patient at the quantified an exposure level for the particular detected extractable/leachable.

The sensitivity of the detector to a given analyte is fixed.  Working within the constraints of the detector’s sensitivity and the dose-based threshold there are a number of variables that can be used by the analytical chemist, when designing the extraction study, to ensure the AET is greater than the method’s limit of detection. Below provides a high-level schematic of these considerations.

The AET calculation

In order, to explore the variables that can be manipulated, to help ensure the AET is greater than the methods LOQ,  it is worth reviewing an example AET calculation for medical devices.

DBT = Dose-Based Threshold (DBT) = Threshold of Toxicological Concern (TTC) as per ICH M7 or  

A = number of medical devices extracted

B = extract volume

C = number of medical devices that contact the body/day

CF = extract concentration factor

UF = analytical methods uncertainty factor 

S= number of sequential extractions (as per 10993-18 exhaustive extraction)

The variables that an analytical chemist can manage to ensure that the AET is greater than an instruments LOQ include:

• Minimizing the solvent volume to sample ratio
• Performing an extract concentration step to increase the analyte concentration. Note: care should be taken to not lose analytes
• Increase the amount of extract that is loaded onto the analytical equipment

Calculating the appropriate AET can be further complicated depending on the test items use and application, defining the appropriate AET is therefore, a collaborative calculation between the analytical chemist and individuals who have knowledge on the clinical use of the product.

The following example highlights how the different use cases can impact the AET for a product which is dosed, to a patient, twice daily from a device which contains 1 dose.

Depending on the devices application the AET can be vastly different depending on whether it is considered a medical device or medicinal product.

This is because the approaches to the extraction may differ between ISO 10993-18 and USP 1663, there are differences in historically accepted SCT/DBTs, and because different regulatory bodies have historically accepted different values for the uncertainty factor.

In the example factors such as the extraction volume and number of devices extracted were kept the same.