Cleaning Validation: glossary of key terms (MACO, ARL, LOD, LOQ…)

If you work in the pharmaceutical industry, you already know that acronyms are a language of their own. We don't just talk about cleaning or residues — we talk about MACO, ARL, PDE, LOD/LOQ, recovery factor… sometimes all in the same sentence.

And as if that weren't enough, each term or acronym conceals critical stakes around patient safety, regulatory compliance, and quality. There is no room to take them lightly.

This glossary offers a clear and concise overview of the key terms in cleaning validation.

MACO – Maximum Allowable Carry Over

This is the maximum quantity of a residue from product A that can be found in product B without risk to the patient. It is indeed a quantity expressed as a mass (not in ppm).

Several calculation methods exist:

• Based on the therapeutic dose (using the minimum dose of A in the daily dose of B, weighted by a safety factor - SF)
• Based on toxicology: via values such as PDE or ADE
• Based on an empirical value (see the definition of 10 ppm)

MACO is the basis of your acceptance criterion but is NOT your acceptance criterion (ARL).

ARL – Acceptance Residue Limit

The ARL corresponds to the maximum acceptable residue limit per unit of surface area. It is a practical translation of the MACO adapted to your equipment train.

10 ppm (of A in B)

The 10 ppm threshold is one of the oldest and most well-known values in cleaning validation. Historically, it was used as a default value to set an acceptable residue limit of product A in product B, in the absence of precise toxicological or therapeutic data.

This threshold was widely adopted because it is simple to apply, easy to calculate, and conservative enough to cover the majority of cases. However, a word of caution: this is an empirical approach, not a universal scientific rule. Today, regulatory authorities (such as EMA or FDA) favour more robust approaches based on toxicological data (HBEL).

AC – Acceptance Criteria

The acceptance criterion in cleaning validation corresponds to the maximum acceptable residue limit on a surface after cleaning, beyond which the process is considered non-compliant.

The ARL is the acceptable limit on your equipment, and the acceptance criterion is its representation in your analytical tube. Several factors must be taken into account:

• The sampled surface area: when you sample 25 cm² or 100 cm², the quantity of residue recovered changes.
• The desorption volume: in the case of a swab, the volume used to bring the collected sample back into solution must be carefully considered. It works against you (= dilution factor).
• The recovery factor: the pitfall to avoid (see below).

The acceptance criterion is simply the conversion of the ARL (mg/cm², µg/cm², …) into a value in an analytical sample (µg/ml, mg/L, ppm, …).

Recovery factor

The recovery factor is something of a truth test: it tells us whether our sampling method is truly capable of collecting what we sample… or whether it misses what matters most.

In practice, it is the percentage of residue recovered from a surface after intentionally depositing a known quantity of product. What was recovered is then compared to what was originally applied.

Tracer content

In some cases, the active substance is not tracked directly, but rather a tracer integrated into the molecule of interest — often a more easily detectable substance (such as a degradation residue or a specific chemical element). The correlation factor, in this context, corresponds to the amount of tracer present in one unit of the molecule of interest.

In other words, it is a conversion coefficient: it allows the measured quantity of the tracer to be translated into the actual quantity of the target substance.

LOD & LOQ

In cleaning validation, it is not enough to say that nothing is visible: you also need to know what your method is truly capable of detecting or measuring.

• LOD (Limit of Detection): this is a bit like saying "there is something here, but I cannot tell how much". The method detects the presence of a residue without being able to estimate it precisely.
• LOQ (Limit of Quantification): this is the threshold from which the method can not only detect, but also measure reliably and repeatably. It is from this point that your acceptance criterion becomes truly actionable.

For your acceptance criteria to hold up, they must always be above the LOQ — otherwise you will never be able to prove they are being met.

PDE – Permitted Daily Exposure

Used in the HBEL approach for calculating MACO, the PDE represents the daily dose that a patient can receive without adverse effects.

HBEL – Health-Based Exposure Limit

This is a generic term encompassing values such as PDE or ADE. These are exposure limits derived from health data, generally determined by a toxicologist based on toxicological studies.

mTD – Minimal Therapeutic Dose

The mTD (or minimum therapeutic dose) is the smallest quantity of an active substance capable of producing a therapeutic effect in a patient. In cleaning validation, the mTD is often used as a reference for setting residue acceptance limits: the goal is to ensure that traces of product A left on equipment before manufacturing product B are well below this minimum dose.

LDD – Largest Daily Dose

The LDD (or maximum daily dose) is simply the largest quantity of a drug that a patient can receive in a single day, according to therapeutic recommendations.

TOC – Total Organic Carbon

TOC is something of the chemical detective in cleaning validation. It measures the total amount of carbon of organic origin present on a surface or in a solution after cleaning. This analytical method does not look for a specific compound, but for anything organic: product residues, excipients, contaminants, and even traces of cleaning agents or detergents.

What makes it particularly useful? Its versatility. TOC is capable of detecting residues even when their exact chemical nature is unknown. It is therefore a non-specific method widely used for biotechnological products in multi-product environments.

Worst-case

A strategy consisting of validating the most unfavourable scenario:

• The most difficult product to clean
• The most difficult equipment to clean
• The most difficult equipment/product combination to clean

This allows the number of validations to be reduced based on scientific justification.

Reproducibility

A fundamental validation criterion. Generally, 3 consecutive compliant runs are required to demonstrate that the cleaning process is reproducible.