Contact Lenses: Which Part of ISO 18369 Applies (ISO 18369)

1. Which part of ISO 18369 applies to your test?

ISO 18369 is a multi-part standard, and protein deposition studies are routinely attributed to the wrong part. It is worth being precise, because a method reference in a technical file is checked.

So Part 3 governs the saline your lens sits in while you measure its power, and Part 4 governs what comes out of the material. Neither specifies a protein-bearing artificial tear fluid, and this is the point that matters most for anyone planning a deposition study.

2. Is there a standardised artificial tear fluid?

The standard saline of ISO 18369-3 is a simple buffered saline. It is the right fluid for conditioning a lens to a defined hydration state, and it contains no protein, no lipid and no mucin, because equilibration is all it is for.

Protein and lipid deposition protocols come from the research literature, not from ISO. Published artificial tear solutions carry salts, proteins, lipids and mucin at tear-relevant concentrations, and are characterised against human tears for pH, osmolality, surface tension and homogeneity. They are well founded, and they differ from one another.

The practical consequence is a documentation one. A deposition result should cite the tear formulation it used, by reference, because there is no normative composition to point at instead. Writing that a lens was tested for protein uptake "according to ISO 18369" is not a method reference an assessor can follow, and it will be queried.

3. Why is lysozyme activity, not concentration, the specification?

Lysozyme is the most abundant protein in human tears and the one that dominates uptake on ionic hydrogel materials, so it is the component a tear fluid exists to deliver. It is also an enzyme, and this is what makes the fluid different from every other product in our catalogue.

A salt solution that has aged has essentially the same composition it started with. A lysozyme solution that has aged has the same concentration and less activity, and the two are not interchangeable. Uptake studies can be run on total protein, in which case concentration is what you need; conformational and denaturation work depends on the fraction still natively folded, in which case activity is the specification and a concentration figure tells you nothing.

Decide which of those two your study is before you order. It changes what the certificate has to state, and it changes how much the cold chain matters.

4. How should a protein-bearing tear fluid be shipped and stored?

The tear fluid is made to order and shipped refrigerated, and the specification we hold ourselves to is that the lysozyme is still active when it reaches you, not merely present. That is a deliberate commitment about the state of the product on arrival rather than on dispatch.

It has a consequence you should plan around: the fluid should be received when the bench is ready for it. A refrigerated protein solution left waiting a fortnight for instrument time has lost part of what you paid for, and no handling at your end can recover it.

If your study needs a lipid fraction as well, say so on the order. Lipid variants change how the fluid must be handled and mixed, and they are not a drop-in substitute for the protein-only formulation.

5. How do you read a lens deposition result?

Uptake is normalised to the lens, so the lens has to be characterised: material, water content, ionicity and surface treatment all change the answer, and an ionic high-water material and a silicone hydrogel are not comparable on a protein figure alone. Extraction efficiency belongs in the report too, since what is measured is what came back off the lens rather than what went onto it.

And the fluid itself belongs in the report: formulation, protein concentration, the activity if it was measured, temperature and duration. Without those, a deposition number cannot be reproduced by anyone, including you in a year's time.

This guide sets out which part of the standard applies and what a tear fluid can and cannot be specified against. It does not reproduce the saline composition or measurement procedures of ISO 18369-3 or ISO 18369-4, which you should obtain from ISO or your national body.