The riboflavin coverage test is a fluorescent tracer method used to verify whether the wash fluid reaches every surface within the test scope, on the load and inside the chamber. A riboflavin solution is applied, a sample cycle defined together with the customer is run, and the equipment is then inspected under ultraviolet light. Any residual fluorescence after the cycle indicates inadequate or unconfirmed coverage at that point and calls for an investigation.

One clarification is needed before going further, because the term can be ambiguous. This article is not about blood testing for vitamin B2, nor about the ophthalmic use of riboflavin in corneal cross-linking. What is discussed here is an industrial test, in which riboflavin is used purely as a fluorescent tracer that makes any residue visible under UV light.

In brief

• What it verifies: that the wash fluid reaches every surface within the approved test scope, on the load and in the chamber. It concerns the accessibility and wettability of surfaces rather than soil removal efficacy.
• Test scope: it should be established through a risk assessment. The outcome of that assessment generally extends beyond the load to the chamber as a whole. In many cases no riboflavin trace is permitted anywhere in the chamber, collection sump included.
• When it is run: preferably during factory acceptance testing, where it can reveal design issues early. The results can support subsequent commissioning and on-site qualification activities. Any repetition at SAT, or following changes to the load, should be defined by the qualification protocol or by change control, based on the impact of the change and the associated risk.
• Tracer: riboflavin, vitamin B2, fluorescent under UV and removable with water under the conditions set by the procedure.
• Result: visual. Residual fluorescence within the test scope is a non-conformity and opens an investigation.
• What it does not replace: soil removal studies, TOC and conductivity monitoring of the final rinse, swab testing, and microbiological challenge where required.

What it is called: coverage test, riboflavin test, spray coverage test

Terminology is not settled, which can create confusion in technical specifications. The expressions most often encountered all refer to the same trial:

• Riboflavin test, which names the tracer
• Coverage test, which names the objective
• Spray coverage test, which names the mechanism

They are operational synonyms. In qualification documents, however, it is worth settling on one and using it consistently across protocol, report and SOP. In an audit, terminological consistency between documents counts for more than the choice of term itself.

What the test demonstrates and what it does not

Coverage is a question of access: did the wash fluid, under the conditions delivered by the test cycle, reach this surface?

Cleaning is a question of removal: was the residue present on this surface taken off it, to below a defined acceptance limit?

The two are sequential, not interchangeable. A machine can demonstrate coverage and still fail a removal study, because reaching a surface is not the same as detaching what is bonded to it. The opposite case matters more in practice: incomplete coverage weakens the evidence that the cleaning process is robust across the whole defined load, because effective cleaning cannot be assumed for areas with inadequate access.

This is why coverage comes first. Starting removal studies before coverage has been established means risking an investigation into detergent chemistry when the real problem is a rack shadowing one face of one or more of the parts to be washed.

The riboflavin test does not demonstrate cleaning efficacy. It provides evidence that the wash fluid reaches the surfaces included in the test scope. Cleaning efficacy must be demonstrated separately, using representative soils, defined residue limits and analytical methods.

The test scope covers the chamber, not only the load

This is where industrial practice most often diverges from the way the test is described, and it is also what generates the most deviations in an audit.

In principle it is true that attention focuses on the load to be washed, but it is increasingly required that at the end of the cycle no trace of riboflavin be detected anywhere in the chamber, water collection sump included, where the risk assessment and the intended use of the equipment identify such surfaces as potential points of product contact, accumulation or retention, and therefore as relevant to cleaning validation, consistent with the principles of Annex 15. The trial can accordingly be set up in two ways, and in practice the second is the rule:

• Accessibility verification only. Used to establish that the fluid reaches every point of the load and of the chamber.
• Verification on a sample cycle. A complete cycle is run to establish that the alternation of rinse, drain, further wash and drain phases removes the riboflavin entirely from all parts in direct contact with the product and with the clean unloading environment.

The second approach answers a concrete risk: avoiding cross contamination when the door is opened. A chamber that washed the load correctly but retains residue in a dead point transfers that residue to the next load, or to the classified environment the door opens into.

Why riboflavin is the tracer of choice

It produces sharp, well-defined fluorescence under ultraviolet light. Riboflavin residue gives a yellow-green glow clearly distinguishable against stainless steel. The signal is discrete rather than a diffuse haze, which makes visual inspection repeatable and objective even between different operators.

It is removed by water alone. The test is straightforward to interpret: under the conditions set by the procedure, riboflavin can be removed by rinsing with water only. Where the water reaches a surface, the residue is eliminated. Where traces of riboflavin remain after the rinse, the wash fluid did not adequately reach that area. The result therefore does not depend on detergent concentration or on contact time.

It has a favourable handling profile and negligible cost. Riboflavin is a vitamin, also used as food colouring E101, and is practical for a trial carried out by hand on production equipment. Preparation and use should nevertheless follow the site’s approved procedure and the applicable safety documentation, and compatibility with the elastomers and surface treatments actually present should be confirmed against the equipment documentation rather than assumed.

Variables and limitations that alter the result

• Light sensitivity. Riboflavin degrades under prolonged exposure to light, so a solution left standing under workshop lighting can lose fluorescence and produce a false conforming result.
• Solution homogeneity. Incomplete dissolution or inconsistent preparation can lead to uneven distribution of the tracer across surfaces. In areas where the tracer accumulates, fluorescence under UV radiation may persist even after rinsing, generating potential false non-conformities. Conversely, in areas reached by insufficient quantities of tracer, the absence of fluorescence is not reliable evidence of process effectiveness, since it may be attributable to inadequate coverage rather than to correct removal of the residue.
• Timing. The intervals between application, cycle and inspection change what is observed, because surfaces dry and residual films redistribute. They should be fixed in the protocol.
• Scope of the answer. Riboflavin is not a surrogate for active ingredient residues, proteins or dried product. It answers a single question: does the water sprayed by the machine reach every surface to be verified?
• Intrinsic fluorescence. Polymers, adhesives, labels and detergent residues from previous cycles can emit their own fluorescence. Before the test, therefore, the fluorescence of the surfaces to be inspected must be checked and documented, so that any areas liable to generate false positives are identified and excluded.

Operating parameters vary between industry publications and between site procedures, so they should be taken from the approved protocol rather than from the literature. Concentrations in the region of 0.2 g/L appear in technical publications, other practice indicates solutions of a different order, and inspection is carried out with a UV source specified by the approved coverage test procedure. Excitation wavelength, observed fluorescence and lamp specification are three distinct quantities and should not be conflated in a protocol.

At LAST Technology the methodological reference is not an external guideline but an internal procedure agreed with the customer, defined jointly during test planning. This approach allows not only verification against the defined acceptance criteria, but also the collection of additional information useful in characterising and assessing the performance of the wash cycle, according to the specific objectives of the test and of the equipment.

Why the test on a washer is not the same as on a process vessel (CIP)

The method is identical. The problem to be solved is not. Industry guidance on automated parts washers is explicit: the parameters to be studied and optimised before qualification include both the design and positioning of the water distribution system and the loading configuration of the parts inside the machine. Coverage is therefore a combined property of the machine and the load to be processed.

The practical consequence is that on a washer the coverage test is not a one-off qualification activity. It is tied to the load. A machine that gave a conforming result with a rack of vials tells you nothing about the same machine with a rack of filling machine components.

Step-by-step procedure on a pharmaceutical washer

1. Define the worst case load configuration

Worst case does not mean the heaviest load or the fullest rack. It means the configuration in which the wash fluid has the greatest difficulty reaching a surface within the test scope: usually the combination of highest part density, the most concave or blind geometries, and the positions most shadowed relative to the delivery points. It must be defined in writing before the trial, with a load map, because a coverage test on an undefined load does not produce repeatable evidence.

2. Record the UV baseline before applying the tracer

Before proceeding with the test, inspect the empty chamber, the rack and the sample load using the same procedure that will be used for the test. Recording this baseline is what allows the operator, an hour later, to distinguish genuine incomplete coverage from a property of the material. It is a step almost nobody performs and it is the cheapest way to avoid an unfounded deviation.

3. Prepare the solution and verify the tracer

Prepare the solution immediately before use, protect it from light and make sure it is completely and homogeneously dissolved. Before running the full trial it is advisable to carry out a preliminary check: apply a small quantity to a clean stainless steel coupon, darken the area and confirm under UV that the fluorescent effect is clearly visible. This check takes only a few minutes and avoids the most insidious risk of the method, a false conforming result caused by an already degraded tracer.

4. Apply the tracer to the load, the rack and the chamber

Apply a thin, even film to all surfaces within the test scope, including the rack and the internal surfaces of the chamber. Evenness matters more than quantity: heavy pooling on a horizontal face creates a reservoir that is still draining at the end of the cycle and can be read as a non-conformity that does not exist.

5. Verify initial tracer coverage under UV, before the cycle

This is the step that makes the whole trial defensible and it is also the one most often omitted. Before starting any cycle, inspect the treated load under UV light. At this stage all surfaces included in the test scope must show clear fluorescence. If a surface shows none, the tracer was not applied at that point; consequently, the absence of fluorescence after the cycle allows no conclusion to be drawn. Document this check with photographs: it is the control that turns the final result into objective evidence.

6. Load the chamber according to the approved map

Load the rack according to the worst case defined in step 1 and photograph the load before closing the door. This photograph documents the actual configuration of the trial. It is also the first element to check should a repetition produce a different outcome.

7. Run the sample cycle defined with the customer

A wash cycle set jointly with the customer is normally run, with alternating rinse, drain, further wash and drain phases. Flow rate, pressure, time, temperature and every other relevant parameter must be documented so that the trial is reproducible.

One choice has to be agreed beforehand: some customers require the riboflavin to be dry when the cycle starts, others do not. Where the dry application is chosen, around one hour is normally allowed after spraying.

8. Inspect under UV, immediately and in the dark

Inspect the load immediately after the cycle, in an environment darkened enough to avoid interference between ambient light and the UV illumination. Work systematically, always following the same sequence defined by the load map, and extend the inspection to the internal surfaces of the chamber and to the collection sump. For internal bores, long tubes and other hard-to-reach geometries, use suitable inspection aids, for example a borescope with a UV source, where direct visual inspection is not sufficient.

9. Record, investigate, correct, repeat

Photograph every area of residual fluorescence, in place, before touching anything. Then investigate the cause, correct it and repeat the complete trial. A partial repetition on the non-conforming area alone does not constitute evidence, because moving one part alters fluid distribution over the neighbouring parts.

Acceptance criteria and test scope

The acceptance criterion must be defined before the trial is executed and stated as follows: at the end of the defined rinse procedure, no residual riboflavin fluorescence shall be observable on any of the surfaces included within the approved test scope.

Attention therefore shifts to the definition of the test scope. A defensible scope must rest on a documented rationale, not on practical convenience. It must include product contact surfaces, surfaces whose contamination could migrate towards those areas, the rack, and the internal surfaces of the chamber together with the collection sump. Any exclusions must be stated explicitly and supported by a documented rationale, never left implicit.

How to read residual fluorescence

Residual fluorescence does not identify a cause. It identifies a point to investigate. Treating it as a direct diagnosis, typically as a blocked nozzle, is the most frequent analytical error in this trial. It is the pattern of what is observed that narrows the field.

Common causes of incomplete coverage and corrective actions

Most of these causes relate to the load and the accessories rather than to the machine itself. The coverage test must therefore be referred to the specific load configuration.

In that context a purpose-designed rack is preferable to a standard rack adapted to the task: it is not a simple support but an integral part of the washing process. It must hold every component in the position, orientation and at the distance required for the water jets to reach all the surfaces to be rinsed. Supports, seats and fixing points must be designed to avoid shadowing, overlap, pooling and contact between components that could compromise coverage. Rack design is therefore a quality activity, to be defined, documented and verified through the coverage test.

Where the test sits within qualification

It is preferable to run the test during factory acceptance testing, as this is the least costly point at which to identify issues that may require modifications or a rack redesign. Results obtained at FAT can also support subsequent commissioning and on-site qualification activities. Any repetition of the test on site during SAT, or following the introduction of a new load type, should be defined within the qualification protocol or change control, based on the impact of the change and the associated risk, and does not therefore constitute a blanket obligation in every case.

Riboflavin compared with other methods

Documenting the trial so that it holds up in an audit

A trial carried out without documentation does not constitute evidence and will not survive review. The minimum set to produce is short.

• An approved protocol before execution, defining the load configuration, the test scope, the tracer parameters, the cycle used and the acceptance criterion.
• The pre-test UV baseline, recording any intrinsic fluorescence observed before the tracer was applied.
• A load map with photographs of the load on the rack, taken before the door is closed.
• UV photographs of the treated load before the cycle, demonstrating that the tracer reached every surface under test.
• UV photographs of the inspection after the cycle, including conforming areas and not only non-conformities.
• A record of deviations, of the investigation, of the corrective actions and of the complete repetition.
• Traceability of the cycle actually executed, which shows the conditions under which coverage was verified.

This last point connects the trial to the wider data integrity framework. What a cycle record should contain, and how it satisfies ALCOA+ principles, is covered in our article on what a cGLP wash cycle record must contain.

How LAST Technology designs washing equipment for spray coverage

Most of the causes of non-conformity described above are design outcomes, not operating errors. This is why LAST Technology treats coverage as an engineering requirement rather than a test to be passed at the end.

The load defines the machine, not the other way round

LAST Technology builds washing equipment around the parts the customer intends to process, from glassware and filling machine components to plastic containers, utensils and large IBC containers. From a coverage-testing perspective this is the decisive factor: a rack designed for the real geometry, with part orientation constrained by the supports and not left to the operator, addresses at source the two most frequent causes of non-conformity, namely shadowing and inconsistent positioning.

The internal rack

The internal rack is not a simple support for the load, it takes an active part in the washing process. Its integrated nozzles are designed to distribute the fluid into shadowed areas, internal cavities and geometries that the chamber nozzles alone would not reach reliably.

When the rack is introduced into the chamber, the system couples automatically to the machine’s fluid distribution network. The connection feeds the rack nozzles, ensuring fluid delivery at the points intended for washing.

Reproducible cycle parameters

A coverage result is only valid for the parameters under which it was obtained. LAST Technology machines run pre-set and custom recipes with continuous control of rinse water TOC, conductivity and pH according to the configuration purchased by the customer, and with controlled injection of chemicals during the wash phase. Because the executed cycle is recorded, the coverage verification can be linked to the conditions that produced it.

Conclusion

The riboflavin test looks trivial: powder, water, a spray bottle and a UV lamp. Its value lies entirely in the discipline around it, namely a worst case load defined in advance, a baseline that rules out intrinsic fluorescence, a verified tracer, a documented check that the tracer reached every surface before the cycle, an inspection that reaches where the eye cannot, and a complete repetition after every correction. All of this serves to answer a single question: did the wash fluid genuinely reach every surface?

On a washer that question is inseparable from the load, which is why coverage is a design problem before it is a validation one. Rack geometry, part orientation, nozzle layout and cycle parameters determine the outcome of the trial long before the trial is run. Designing them together, around the parts that will actually be processed, is what customisation means in practice and it is how LAST Technology specifies its washing equipment. To assess coverage for your parts and geometries, see our UCW TYPE – Glassware/ components cGMP washing equipment, or contact our technical team.

Sources and further reading

• ISPE, Pharmaceutical Engineering, “Master Soil Selection for Cleaning Validation of Parts Washers”, May-June 2021.
• ISPE, Sterile Products Processing Community of Practice, “Validation of an Automated Parts Washer”.
• ISPE, Sterile Products Processing Community of Practice, “Validation of an Automated Glassware Washer”.
• VDMA, information sheet on the riboflavin test for low-germ and sterile process technologies.

Massimo Castellarin President & CEO