Critical Quality Attributes in Tablet Compression

In short: At compression, the blend becomes a tablet, and several quality attributes are fixed for good: weight, thickness, hardness, friability, content uniformity, disintegration and dissolution. This page explains what a critical quality attribute (CQA) is, how each tablet attribute links to press settings, and how to monitor them. Limits come from the pharmacopoeia, the registered specification or an in-house standard, so check yours.

What is a critical quality attribute?

ICH Q8(R2) (Pharmaceutical Development) defines a critical quality attribute as a physical, chemical, biological or microbiological property or characteristic that should be within an appropriate limit, range or distribution to ensure the desired product quality. A critical process parameter (CPP) is a process parameter whose variability affects a CQA, so it should be monitored or controlled to make sure the process delivers the desired quality.

The thinking runs in one chain: the quality target product profile defines what the product must do, the CQAs describe the properties that deliver it, and risk assessment (see quality risk management) links each CQA to material attributes and process parameters. Which attributes are critical is a product-specific decision recorded in your development and validation documents, not a fixed list.

Compression: where the attributes are set

On a rotary press, the die fill, the pre-compression and main compression forces, the turret speed, the feeder speed and the punch penetration all act on the powder. Material properties such as flow, particle size and moisture also matter. The same settings that control one attribute often affect another, so they are best studied together.

Attributes to monitor and what drives them

Attribute	What it is	Main drivers at compression	Why it matters
Tablet weight and weight variation	Mass of individual tablets and spread across a batch	Die fill depth, blend flow, feeder and turret speed, tooling wear	Dose accuracy when drug content is uniform in the blend
Thickness	Tablet height	Fill and compression force, tooling	Packaging fit, appearance, link to hardness
Hardness (breaking force)	Force needed to break a tablet	Compression force, fill, lubricant level, granule properties	Handling strength versus disintegration and dissolution
Friability	Weight loss when tablets are tumbled	Compression force, granule bonding, tablet shape	Resistance to chipping and abrasion in coating, packing and transport
Content uniformity	Spread of drug content between tablets	Blend uniformity, segregation in the hopper and feed frame, fill variation	Each tablet must deliver the intended dose
Disintegration and dissolution	Break-up time and drug release	Hardness and porosity, lubricant, disintegrant	Drug release and bioavailability

Weight variation

Tablets are sampled at set intervals and weighed individually, and the results are compared with the limits for the product. Weight variation can reflect variable die fill from poor flow, segregation, feeder problems or worn tooling. It is meaningful for dose accuracy only when the blend itself is uniform, which is why content uniformity is checked separately.

Hardness

Hardness is measured as the force needed to break the tablet, normally expressed in newtons (N) or kilopond (kp), not in kilopascals. Harder tablets resist handling but may disintegrate and dissolve more slowly. Softer tablets may chip, cap or laminate. Set the working range from development data for each product, and read the hardness together with thickness and weight, since all three move together when fill or force changes.

Friability

Friability is tested by tumbling a weighed sample of tablets in a friabilator, removing dust and reweighing. The weight loss is stated as a percentage of the starting weight. Typical pharmacopoeial practice for uncoated tablets accepts a small loss, and tablets that cap or split in the test are generally unacceptable.

Content uniformity

Content uniformity is demonstrated by assay of individual units or, where permitted, by weight variation, following the pharmacopoeial chapter on uniformity of dosage units (for example USP <905>, Ph. Eur. 2.9.40). Compression can worsen uniformity when the blend segregates in the hopper or feed frame, so sampling at the start, middle and end of the run, and after stoppages, is good practice.

Disintegration and dissolution

Disintegration and dissolution are usually finished-product tests, but compression settings strongly influence them. Many sites check disintegration as an in-process test, and use dissolution to confirm the chosen hardness range. See dissolution test apparatus operation and calibration and disintegration tester operation and calibration.

Setting limits and controls

Pharmacopoeial limits. Friability, uniformity of dosage units, disintegration and dissolution follow the pharmacopoeia and the registered specification. Always use the current edition for your market.
In-process limits. Weight and hardness ranges at the press are usually tighter than the final specification, so that trends are caught early. These are example in-house criteria, set from development and validation data, not pharmacopoeial requirements.
Frequency. The checking frequency should be justified in the batch record and validated. For example, an in-house practice might be weight checks at fixed intervals and hardness and thickness checks at a less frequent interval. These intervals are an example in-house criterion, not a regulatory number.
Equipment. The press and test instruments must be qualified and calibrated. See qualification stages and the operation and cleaning SOP for a compression machine.
Validation. Ranges for the CPPs and the sampling plan are confirmed in process validation.

Typical problems and the likely cause

Observation	Common cause to investigate
Weight varying between tablets	Poor flow, uneven die fill, segregation, worn punches or feeder fault
Capping or lamination	Excess air in the powder, over-compression, too much fines or lubricant
Sticking or picking	Moisture, tooling surface, insufficient lubricant
Low hardness with high friability	Low compression force, weak binder action
Slow disintegration	Over-hardness, lubricant over-blending, disintegrant level

Frequently asked questions

Are weight, hardness and friability always CQAs?

Not automatically. ICH Q8(R2) leaves the decision to product-specific risk assessment. Many products treat them as critical or as controls on a CQA such as content uniformity or dissolution.

Is hardness a CQA or a process parameter?

It is a measurable attribute of the tablet, so it can be an in-process control or a CQA. Compression force is the process parameter that mainly moves it.

What should happen if in-process results go out of range?

Stop or adjust the press as the batch instructions say, check tablets back to the last good check, and investigate under your deviation procedure. Do not release the affected tablets until the investigation is closed.

How long must compression records be kept?

Batch and in-process records are kept for at least 1 year after the expiry date of the respective product batch (EU GMP Part I Chapter 4 section 4.11, or 5 years after QP certification if longer; 21 CFR 211.180(a)), and longer if local law or the marketing authorisation requires.

References

ICH Q8(R2), Pharmaceutical Development, Step 4 (August 2009): glossary definitions of critical quality attribute and critical process parameter.
European Commission, EudraLex Volume 4, Part I, Chapter 4: Documentation, section 4.11.
US FDA, 21 CFR 211.180(a), General requirements (records retention).
USP general chapters on tablet friability (1216), tablet breaking force (1217) and uniformity of dosage units (905); Ph. Eur. general methods 2.9.7, 2.9.8 and 2.9.40. Current editions apply.