Rothwell, AUM and Catheter Encrustation Fluid: Artificial Urine Terminology

1. Why does “artificial urine” mean three different fluids?

“Artificial urine” is used in the literature and in supplier catalogues for at least three chemically distinct fluids, built for three different tests. Ordering the wrong one does not merely waste a bottle: it produces a result that looks plausible and means nothing, because the fluid was never designed to answer the question being asked.

The confusion runs in one particular direction. Because some supplier material loosely calls an absorbency test an “AUM” test, a search for AUM often lands on hygiene products when the underlying need is a urological device study. The two fluids are not interchangeable: The ISO 11948 fluid deliberately carries little calcium, magnesium phosphate or citrate, because those species would encourage exactly the crystal formation an absorbency test has no interest in. The AUM fluid is built around them, because encrustation is the point.

2. What does the ISO 11948-1 Rothwell fluid contain?

ISO 11948-1 and EDANA NWSP 010.1 specify the standardised determination of total absorption capacity and strike-through time for incontinence and hygiene products. The name Rothwell method comes from the original repeatability and reproducibility work published under that name, and is now used interchangeably with the standard number in the industry, someone searching “Rothwell test” and someone searching “ISO 11948” want the same fluid.

In outline: the dry specimen is weighed, conditioned, then fully submerged in synthetic urine at pH 6.20 ± 0.20. After draining under controlled conditions it is weighed again, and capacity is expressed as grams of fluid retained per gram of dry product. Because a campaign consumes litres rather than millilitres, this is normally ordered in cubitainers rather than bottles.

Worth passing on to anyone benchmarking products: a laboratory Rothwell figure is a controlled-condition maximum, not an in-use expectation. The working rule in the field is that practical absorption is around a third of it. And the differences between laboratories running the same apparatus are systematic rather than random, one published comparison finding two laboratories exceeding a third by about 13 % and 8 %, which is larger than most of the product-to-product differences anyone argues about. Compare products tested in the same laboratory, or compare nothing.

3. When do you need AUM rather than a plain synthetic urine?

Catheter work needs a substantially richer fluid, because the question is not how much a material absorbs but how quickly a device crystallises and blocks. Formulations of the Griffith and Stickler type, and the variants circulating as “Artificial Urine Medium”, typically carry urea, creatinine, uric acid, citrate and oxalate alongside the calcium, magnesium and phosphate that drive struvite and calcium-phosphate deposition on an indwelling device.

Many protocols additionally inoculate the fluid with a urease-producing organism, commonly Proteus mirabilis, so that pH rises over the incubation the way it does in a colonised bladder. That pH rise is what actually drives encrustation, not the baseline chemistry on its own. For long flow-rig studies a sterile fluid matters, because unintended contamination changes the pH trajectory and therefore the result.

4. Is the standard you cite still current?

If your technical file still cites EN 1616, check it before your next audit. EN 1616:1997 was withdrawn: urethral catheters are now covered by EN ISO 20696:2018, and drainage catheters (ureteral, nephrostomy, suprapubic) by the separate ISO 20697:2018. We have aligned our own references accordingly. Whether the fluid specification itself changed between the old and the new texts is a question worth putting to your device-testing laboratory rather than settling by assumption in either direction.

5. What do buyers ask us most often about artificial urine?

Is Rothwell testing the same as the “synthetic urine” sold for drug-screen kits?
No, and the two should never be conflated in a technical context. ISO 11948 synthetic urine is a simple, standard-referenced electrolyte solution for testing absorbent materials. The drug-screen product is formulated to imitate the physical checks applied during a screening and is built to no testing standard at all. We do not manufacture or supply it.

I need artificial urine for a catheter encrustation study, is that ISO 11948?
No. ISO 11948 is an absorbency method and its fluid lacks the calcium, magnesium phosphate, citrate and oxalate chemistry an encrustation study depends on. You want the AUM fluid.

What does “AUM” mean when I see it in a catalogue?
Read what test is being described. If the surrounding text mentions incontinence pads or absorption capacity, it almost certainly means ISO 11948. If it mentions catheters, encrustation, biofilm or urological devices, it means the Griffith/Stickler-type formulation.