Leprosy Mailing List – August 16, 2013
Ref.: (LML) SW monofilaments and ball point pen
From: Wim van Brakel, Royal Tropical Institute, Amsterdam, the Netherlands
Dear Pieter,
I would like to respond to the interesting discussion about use of the monofilament test (MFT) versus the ball point pen test (BPT). I will address three questions: what is the purpose of the these two tests, what scientific evidence exists of the reliability of either test, and what considerations should made regarding their use?
1. Purpose
Both tests are intended for screening of peripheral sensory nerve function. Neither was intended for testing sensation of skin lesions or for diagnosing leprosy. I don’t know of any studies that have compared these tests with the cotton wool test in terms of sensitivity, specificity and predictive value, but the consensus is that (a ‘wisp’ of) cotton wool is the preferred tool to test sensibility of skin lesions. It is obvious that the variability in pressure delivered by a touch or stroke of cotton wool would be much less than a touch with a stiff object such as a ballpoint pen. A thin monofilament (MFT) may be used effectively also (e.g. the green 50 mg filament), but this was not its original purpose. The INFIR cohort study found a good correlation between neurophysiological measures and MFT results, but this referred to nerve trunk lesions only; not to sensory impairment of skin lesion.
The ball point pen test (BPT) is not intended to be a replacement of the MFT, but to be a rapid screening test. I realize the very real operational constraints described by Dr. Cross, which may preclude the use of the MFT. However, the optimal procedure would be to conduct a quick BPT as a routine screening procedure during, and if necessary also in the first few years after MDT. If a patient does not feel the touches of the ballpoint pen adequately, s/he should be tested with the MFT. The MFT provides a semi-quantitative assessment of sensibility, which is much more suitable for monitoring over time than the BPT, which essentially only provides a ‘Yes/No’ answer. In practice, most leprosy programmes only use the BPT. This is much better than no test at all and should be seen as the minimum quality standard for prevention of disabilities. I would recommend adding the MFT, where possible, since this gives the health worker a simple, relatively inexpensive and more accurate means of monitoring the nerve function of his/her patients. The kind offer of Mr. Robert Jerskey to supply calibrated monofilaments to those who are interested at the ILC could be a step in the right direction for some. Of course, a sustainable and low-cost source needs to be established, especially in Africa and Asia.
2. Reliability
I agree with Dr. Cross that comparative reliability is an academic question if one of the two tests is not available. However, if the BPT would not be sufficiently reliable, then its use should not be recommended at all, even if no alternative were available. The inter-tester reliability of the MFT has been demonstrated repeatedly and is very good, with weighted kappa values usually exceeding 0.8 (i.e. chance-corrected agreement between the testers is 80% or better). Below are some of the publications; these refer only to the leprosy field, but others have found the same.
Readers will notice that, fortunately, the BPT was also found to have very acceptable reliability, with coefficients only slightly below those of the MFT. Good news, therefore, for the many programme that currently have no alternative available.
3. Considerations for use
The difficulty with the BPT is the consistency of the technique used. As Dr. Schreuder wrote as an editor’s note in Dr. Warren’s earlier contribution to this discussion, the correct technique is a very light touch, preferably one that does not produce any depression of the skin at all. This was published as consensus advice 10 years ago:
Watson JM, Lehman LF, Schreuder PA, van Brakel WH. Ballpoint pen testing: light touch versus deep pressure. Lepr Rev 2002; 73(4):392-393.
So, we should definitely not use the weight of the pen; even a light pen (e.g. the 4-g BIC pen) is too heavy to detect early sensory impairment. The threshold for normal sensation is measured in milligrams, rather than grams. In my experience, it is much easier to perform the test and control the pressure when holding the pen at an angle (~45 degrees) than perpendicular, particularly when testing the sole of the foot.
In this context, it may also be worth noting that a stroking movement (either with cotton wool or a filament) provides a much stronger sensory stimulus than a static touch. A static touch may be preferred for that reason, but the most important point is to be consistent in the method and use either static touches or gentle stroking, but not a mixture.
Monofilaments need replacing after varying lengths of time (as soon as the filament remains bent after testing). This is another reason why a sustainable source of standardised monofilaments is needed. It ought to be possible to develop this, since the MFT is also recognised as a test of choice to monitor diabetes-related neuropathy. This is a rapidly growing problem in many leprosy-endemic countries, which should ensure a future market for cheap, high-quality monofilament test kits.
With kind regards,
Wim van Brakel
Technical Advisor NLR
van Brakel WH, Khawas IB, Gurung KS, Kets CM, van Leerdam ME, Drever W. Intra- and inter-tester reliability of sensibility testing in leprosy. Int J Lepr Other Mycobact Dis 1996; 64(3):287-298.
Anderson AM, Croft RP. Reliability of Semmes Weinstein monofilament and ballpoint sensory testing, and voluntary muscle testing in Bangladesh. Lepr Rev 1999; 70(3):305-313.
van Brakel WH, Khawas IB, Gurung KS, Kets CM, van Leerdam ME, Drever W. Intra- and inter-tester reliability of sensibility testing in leprosy. Int J Lepr Other Mycobact Dis 1996; 64(3):287-298
LML - S Deepak, B Naafs, S Noto and P Schreuder
LML blog link: http://leprosymailinglist.blogspot.it/
Contact: Dr Pieter Schreuder << editorlml@gmail.com
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