Leak Rate Measurement for Pharma Isolators: What You Need to Know!

Those professionals who involve working with barrier system in aseptic manufacturing or containment system for Highly Potent APIs (HPAPIs) and Radio-pharmaceuticals know the critical importance of ensuring proper leak tightness of isolators or other enclosure systems. This article intends to highlight important points in this regard. ?

Leak Rate vs Leak Detection

First, let's clarify the difference between leak rate and leak detection. Leak rate is the amount of air lost from a positive isolator (or gained by a negative one) per unit time, often expressed as percentage volume loss per hour however leak detection refers to methods for locating specific points of leakage after a failed leak rate test.

Relevant Standards & Guidelines

ISO 14644-7 and 10648-2 define four leak rate classes from very tight; Class 1 (≤0.05% vol/hr) to leaky; Class 4 (≤10% vol/hr).

As a rule of thumb:

• Class 1: Biological safety cabinets (BSC) Class III (not really relevant to common pharmaceutical isolators)
• Class 2: Negative pressure isolators used for working with HPAPIs
• Class 3: Positive pressure aseptic isolators
• Class 4; Class 4. Not appropriate for pharmaceutical isolators.

It should be reminded that the Royal Pharmaceutical Society's QA of Aseptic Services also recommends ≤0.25% for negative isolators and ≤1% for positive isolators.

PDA- TR 34 ?point outs ?that American Glove Box Society specifies a leak rate of not more than 0.5% of the isolator volume per hour.

Leak Rate Measurement Methods;

There are different methods used as follows;

• Pressure Hold: Supplies airflow to hold constant pressure, with the leak rate read directly from the flow meter. Suitable for larger/leakier isolators.
• Nitrogen Dilution: Tracks oxygen ingress into a nitrogen-filled isolator using a sensitive oxygen meter. Sensitive but less practical.?
• Pressure Decay is the most common, measuring pressure drop over time in a sealed positive isolator. Nevertheless, some users may test negative isolators at positive pressure, arguing that this represents the worst case scenario for leakage.
• Other techniques like Parjo/Fosco methods (using frictionless pistons) are rarely used for pharma isolators.

?When should Leak Rate Measurement take place?

• Factory Acceptance Test (FAT)
• Site Acceptance Test (SAT)
• Operational Quali?cation (OQ)
• Planned Preventative Maintenance (PPM)
• Following any maintenance or repair which has breached the sealed structure

Pressure Decay Calculations;

When it comes to leak rate measurement of pharmaceutical isolators (Class 2 and 3), the pressure decay method is most widely used. Let's dive into some detailed examples of how to calculate the allowable pressure decay and leak rates using the key equations:

• Leak rate (% volume loss per hour);

? L = PD x 6000 / (SP x M) (Equation I)

Where:

L = Leak rate (% volume loss per hour) PD = Pressure decay (Pa)

SP = Starting pressure (Pa) = 101325 + test pressure

M = Test time (minutes)

• Maximum allowable pressure decay for a target leak rate class;

PD = L x SP x M / 6000

Example 1: Calculating Leak Rate Let's say an isolator is tested at 200 Pa for 5 minutes and the measured pressure decay is 19 Pa. Using Equation I:

L = 25 x 6000 / [(101325 + 200) x 5] = 0.23% volume loss per hour

So the measured leak rate is 0.23% vol/hr, just meeting Class 2 (≤0.25% vol/hr).

Example 2: Calculating Allowable Decay You need to test for Class 3 leak rate (≤1% vol/hr) with a 10 minute test at 150 Pa starting pressure. Using Equation II:

PD = 1 x (101325 + 150) x 10 / 6000 = 169 Pa

This means the maximum allowable 10 minute pressure decay to pass a Class 3 test at 150 Pa is 169 Pa.

However, 169 Pa decay may be too high for some isolators.

Let's revise to a 5 minute test:

PD = 1 x (101325 + 150) x 5 / 6000 = 84 Pa

So for a 5 minute Class 3 ≤1% test at 150 Pa, the maximum allowable decay is 84 Pa to pass the test.

For Class 2 ≤0.25% over 10 min at 150 Pa, the maximum allowable PD is 42 Pa to pass the test.

The Royal Pharmaceutical Society's QA of Aseptic Services also recommends decay rate of 4.2 and 17 Pa per minute for negative isolators and for positive isolators respectively.

These examples illustrate how the two key equations can be used to calculate leak rates from measured pressure decays, or to determine the allowable decay for a target leak rate class, test pressure, and duration.

Some key practical points which should be considered during the test;

• Test pressure is typically 2-3 times working pressure (e.g. 150 Pa for 50 Pa operating), A successful result will ensure that the leak rate at working pressure is better than the speci?ed class.
• 5 to 10 minute tests are common to balance quickness and repeatability.
• The sleeves should be turned inside out i.e. allow the internal pressure of the isolator to push them completely out of the isolator in the case of positive pressure isolators or to pull them into the isolator in the case of negative isolators.
• ?Environment of surrounding area should not be changed quickly for example, to doors opening or HVAC systems readjusting.
• It's crucial to ask your isolator supplier which leak rate class and standards their products comply with.
• Refer to ISO 14644-7 or 10648-2 or other relevant guidelines for guidance on generally accepted leak rate classes.
• Proper training is highly recommended for all isolator operators and validation personnel involved in leak testing.
• If the leak rate test shows that the isolator does not meet the speci?ed class, then a leak detection method should be done to track down the source, or sources of the problem.
• Leak rate measurement is a relatively simple process but even so, some training for test engineers will give quicker and more reliable results.

References;

• ?Leak rate measurement for pharmaceutical isolators: Practical guidance for operators and test engineers, Tim Coles; Clean Air and Containment Review, Issue 11, July 2012
• ?BS EN ISO 14644-7:2004, Cleanrooms and associated controlled environments, Part 7: Separative devices (clean air hoods, glove-boxes, isolators and mini-environments)
• BS EN ISO 10648:1994, Containment enclosures, Part 2: Classi?cation according to leak tightness and associated checking methods
• ?Royal Pharmaceutical Society, Quality Assurance of Aseptic Preparation Services: Standards Part A, ?Fifth edition
• ?PHSS Clarity In GMP Guidance Pharmaceutical Isolator Leak Integrity Classes-?PDA, Technical Report R-34; Validation of Isolators Systems, Volume 55, No.5, 2001