Technical note
An investigation of the repeatability and reproducibility of ISO 11948-1 (the Rothwell method) for measuring the absorption capacity of incontinence pads

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Abstract

The repeatability and reproducibility (precision within and between laboratories, respectively) of an international standard method (ISO 11948-1, the Rothwell method) for measuring the absorption capacity of incontinence pads was investigated. The 74 shaped disposable bodyworn insert pads for heavy incontinence on the UK market in spring 1997 were tested in three laboratories experienced in using the method, one in each of England, Spain and Sweden. Coefficients of variation (standard deviation as a proportion of the mean) for five repeats rarely exceeded 5% within any laboratory. However, there were systematic differences between laboratories: results from the Swedish and Spanish laboratories typically exceeded those from the English laboratory by 13% and 8%, respectively. The good repeatability suggests that the method is capable of adequate precision but the poor reproducibility implies that the instructions in the standard for building and/or using the test apparatus are inadequate, leaving too much room for interpretation. Having studied the data presented here and viewed videos of the apparatus in use in five laboratories (including the three contributing to this note) the ISO working group which wrote the original standard has identified several likely sources of imprecision and is now working to revise the standard to improve its reproducibility.

Introduction

Incontinence pad purchasers consider many factors in deciding which products to buy [1] but the ability of a pad to receive and retain urine with an acceptably low risk of leaking is a fundamental requirement. Establishing the leakage performance of products is, however, far from trivial. Clinical evaluations are expensive and time-consuming and there are numerous products to choose from. Furthermore, manufacturers modify their designs frequently and so published data exist on very few of the products available at a given time.

Laboratory tests are generally much easier to run and many methods have been devised, each seeking to measure some characteristic(s) of a pad thought to relate to its leakage performance in real use. In the early 1990s a plethora of such methods was available but none was widely accepted, not least because none had been clinically validated. In response, an International Standards Organisation working group (ISO TC173/SC3/WG2) was set the task of determining the best available methods and capturing them in international standards. It began by running a large international multi-centre project [2] focusing on the product category responsible for most expenditure: disposable bodyworn pads for heavily incontinent users. The strategy was to invite any laboratory claiming to have a promising method to run it on a specified range of products which were also subjected to clinical evaluation to measure their leakage performance. More than 50 laboratory methods were offered and each was evaluated against the following criteria to determine which was best. The method should:

  • Provide data correlating strongly with the leakage performance of pads in clinical use.

  • Provide data with good repeatability and reproducibility (precision within and between laboratories, respectively).

  • Involve simple, inexpensive equipment.

  • Be quick and easy to perform.

The method which emerged as the best1 was published as an International Standard (ISO 11948-1) in 1996 [3] and is now in widespread use. However, pad design has developed a great deal in recent years and it is timely to check that the criteria against which the method was originally selected hold true for modern pads. Two causes for concern have been voiced. Firstly, there is anecdotal evidence that the method may have poor reproducibility for modern products. Secondly, since pads now have far more complex structures than those used for the original work on the standard, it might be that correlation between clinical and laboratory test data is no longer adequately strong. This note addresses the first concern while a future paper will address the second.

Section snippets

Materials

The 74 products selected for the work were those used in a clinical evaluation of all the shaped disposable bodyworn insert pads for heavy incontinence on the UK market in spring 1997 [4]. Such pads are usually held in place with stretch mesh pants. A typical pad is shown in Fig. 1 while full details of each of the 74 products are given elsewhere [4]. All the products comprised an absorbent core sandwiched between a water-proof backing beneath, and a water-permeable topsheet or coverstock next

Method

The method described in ISO 11948-1 was used to test the 74 products in each of three laboratories experienced in its use: the Pharmacy Department at the Cumberland Infirmary, Carlisle, England; the Swedish Handicap Institute in Stockholm, Sweden; and the Instituto Salud Carlos III in Madrid, Spain.

ISO 11948-1 involves measuring the total absorption capacity of a pad under standard conditions using the apparatus shown in Fig. 2. A pad is weighed dry and placed (water-poof backing uppermost) on

Results

The repeatability of the method (precision within laboratories) was investigated by calculating the standard deviation as a proportion of the mean (= the coefficient of variation) for the five repeat measurements for each product in each of the three contributing laboratories. The results are summarized in Fig. 3. The coefficient of variation was less than 5% of the mean for 82% of test runs and in only 1% of runs did it exceed 10%.

The reproducibility of the method (precision between

Discussion and conclusions

Although the repeatability of ISO 11948-1 (precision within laboratories) was very good, with coefficients of variation only rarely exceeding 5% for five repeats, reproducibility (precision between laboratories) was less impressive, with substantial systematic differences between the three laboratories contributing data. Video recordings made in the laboratories revealed three areas in which differences of interpretation are likely to have had a major detrimental effect on test reproducibility.

Acknowledgements

The discussion presented in this note is based on a meeting of ISO working group ISO TC173/SC3/WG2 in Stockholm in May 2001 at which the data were debated and the videos reviewed. We are very grateful for the contributions of the working group, under the chairmanship of Dr Jim Abbott. We also wish to thank the UK Medical Devices Agency which funded some of the experimental work.

References (4)

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