Light obscuration, microscopic counting and the limits applying to each volume
What EP 2.9.19 is
EP 2.9.19 is the European Pharmacopoeia general chapter dedicated to particulate contamination by sub-visible particles in injections and infusions.
The definition adopted in the chapter is precise and delimits scope: particulate contamination consists of extraneous, mobile, undissolved particles, other than gas bubbles, unintentionally present in the solutions. Gas bubbles do not count. Dissolved material does not count. What is counted is unwanted, insoluble particulate matter.
Sub-visible does not mean irrelevant. Particles below the visual detection threshold can indicate filtration failure, packaging component degradation, formulation incompatibility or protein aggregation, all with direct clinical implications in a parenteral product.
Where the test applies
The chapter covers most small and large volume parenteral products, including ready to use solutions for intramuscular, subcutaneous and intravenous administration.
Beyond solutions for dialysis, the European Pharmacopoeia requires this test for:
- Liquid parenteral preparations (monograph 0520).
- Sterilised water for injections (monograph 0169).
- Solutions for organ preservation (monograph 1264).
Not all parenteral preparations can be examined by one or both of these methods, and the chapter acknowledges this explicitly.
Method 1: light obscuration particle count
Method 1 is the light obscuration particle count test, and it is the chapter's preferred procedure.
The principle is direct: the sample passes through a cell where a light beam is partially obscured by each particle. The magnitude of obscuration correlates with particle size, and the number of events yields the count.
The classic procedure calls for removing four portions of not less than 5 mL each and counting particles equal to or greater than 10 µm and equal to or greater than 25 µm. The result from the first portion is disregarded, and the mean of the remainder is calculated.
Disregarding the first portion is not bureaucratic detail: it carries system carryover and initial sample handling effects, and including it would bias the result upward.
Method 2: microscopic particle count
Method 2 is the microscopic particle count test. It uses a suitable binocular microscope, a filter assembly for retaining particulate contamination and a membrane filter, with a circular diameter graticule for sizing.
It comes into play in two distinct situations:
- When Method 1 is not applicable, for instance with preparations having reduced clarity or increased viscosity.
- When the Method 1 result exceeds the limits: in that case the preparation is tested by the microscopic particle count test.
That second case is frequently misunderstood. Exceeding the limit under Method 1 is not automatic failure. It is the trigger for Method 2, and reaching a conclusion on conformance may require running both procedures in sequence.
The technical reason is well known: light obscuration is sensitive to gas bubbles, refractive index differences and translucent particles, which can produce both false positives and underestimation. Microscopic counting observes the particle directly.
Sample preparation under Method 2
The chapter details preparation according to unit volume:
- Large volume parenterals: single units are tested.
- Small volume parenterals under 25 mL: the contents of 10 or more units are combined in a cleaned container. Where justified and authorised, the test solution may be prepared by mixing the contents of a suitable number of vials and diluting to 25 mL with particle-free water R, or with an appropriate solvent without particle contamination when particle-free water R is not suitable.
- Small volume parenterals of 25 mL or more: may be tested individually.
Combining units has a statistical consequence often overlooked: it dilutes the effect of an out of pattern unit, which suits the purpose of the test but does not replace investigation where a discrete process failure is suspected.
Limits and evaluation criteria
Evaluation depends on the nominal container volume:
- For preparations supplied in containers with a nominal volume greater than 100 mL, the criteria of test 1.A apply.
- For preparations supplied in containers with a nominal volume less than 100 mL, the criteria of test 1.B apply.
- For preparations supplied in containers with a nominal volume of 100 mL, the criteria of test 1.B apply.
The case of exactly 100 mL nominal volume deserves attention, since much of the misapplication happens there. Worth noting: for nominal 100 mL parenteral products, the EU considers the testing criteria from all three harmonised pharmacopoeias acceptable.
Under test 1.B, applying to containers of nominal volume of 100 mL or less, the preparation complies if the average number of particles in the units tested does not exceed 3000 per container equal to or greater than 10 µm, and does not exceed 300 per container equal to or greater than 25 µm.
Harmonisation: EP 2.9.19, USP 788 and JP 6.07
This chapter has undergone pharmacopoeial harmonisation conducted by the Pharmacopoeial Discussion Group (PDG), and that is among the most practically valuable facts for multinational operations.
The equivalent texts are:
- Ph. Eur. 2.9.19, Particulate Contamination: Sub-visible Particles.
- USP 788, Particulate Matter in Injections.
- JP 6.07, Insoluble Particulate Matter Test for Injections.
According to Annex 3(R1) of the ICH Q4B guideline, these official texts can be used as interchangeable in the ICH regions, subject to one condition: instrument calibration and system suitability measurements should follow regional good manufacturing practice requirements.
There is, however, a recorded caveat that operations exporting to the United States need to know: the FDA might request that a company demonstrate the chosen method is acceptable and suitable for a specific material or product, irrespective of the interchangeability declaration.
PDG revision in progress
A revised version of chapter 2.9.19 was published in Pharmeuropa 33.2 and released for public consultation. The text resulted from lengthy discussions among experts from the Japanese Pharmacopoeia, the United States Pharmacopoeia and the European Pharmacopoeia within the PDG, with all three pharmacopoeias putting the updated text out for public consultation in their respective regions.
One technically relevant point of the revised text: the light obscuration particle count test now allows sample volumes less than 5 mL, depending on instrument capability.
That change connects directly to an earlier adaptation. At the 165th session of the European Pharmacopoeia Commission, in November 2019, revisions were adopted that supplemented the PDG harmonised text with alternative local requirements applicable to biological parenteral preparations.
The motivation is concrete: biological preparations are supplied in low volumes. The local requirements, marked in the text with white diamonds, allow these and other preparations to be tested using volumes smaller than 5 mL where suitable instrumentation is available.
Sub-visible and visible particles: distinct chapters
EP 2.9.19 addresses sub-visible particles. Visible particles are covered in their own chapter, 2.9.20.
Alongside it, the European Pharmacopoeia adopted general chapter 5.17.2, non-mandatory, providing recommendations on testing of particulate contamination by visible particles. That chapter addresses:
- The different sources of foreign particle contamination in liquid preparations.
- Inspection stages during production and quality control, including stability testing.
- Acceptable quality level (AQL) testing, with reference to ISO standard 2859-1, following 100% inspection of the batch.
A relevant principle recorded in that chapter: detection of visible particles in parenteral products is probabilistic in nature, and the occurrence of particles is random. This means 100% inspection does not equate to a guarantee of absence, and the control strategy must account for that statistical reality.
Practical implications for the laboratory
Running EP 2.9.19 in a way that holds up at audit takes more than operating the particle counter. The points most often generating observations are:
- Instrument calibration with traceable standards and system suitability per regional GMP requirements.
- Control of the testing environment, to avoid particle contribution from the laboratory itself.
- Quality of the particle-free water used for dilution and blanks.
- Degassing technique and handling, to avoid counting bubbles as particles.
- Recording the disregarded first portion and the calculated mean.
- A defined procedure for triggering Method 2 when Method 1 exceeds limits.
- Documented justification when applying the reduced volume local requirement.
For operations involving particle characterization and counting, calibration and equipment qualification, this set defines the real scope of implementing the test.