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Accuracy of Monofilament Testing to Diagnose Peripheral Neuropathy

The purpose of the study is to summarize evidence about the diagnostic accuracy of the 5.07/10-g monofilament test in peripheral neuropathy.

Researchers at the University of Amsterdam, Netherlands, and the Erasmas University Rotterdam, Netherlands, conducted a systematic review of studies in which the accuracy of the 5.07/10-g monofilament was evaluated to detect peripheral neuropathy of any cause using nerve conduction as reference standard. Methodological quality was assessed using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) tool.

Jacquelien Dros, MD, Astrid Wewerinke, MD, Patrick J. Bindels, MD, PhD, and Henk C. van Weert, MD, PhD, reviewed 173 titles and abstracts of articles to identify 54 potentially eligible studies, of which 3 were finally selected for data synthesis. All studies were limited to patients with diabetes mellitus and showed limitations according to the QUADAS tool. Sensitivity ranged from 41% to 93% and specificity ranged from 68% to 100%. Because of the heterogenous nature of the studies, a meta-analysis could not be accomplished.

Despite the frequent use of monofilament testing, little can be said about the test accuracy for detecting neuropathy in feet without visible ulcers. Optimal test application and defining a threshold should have priority in evaluating monofilament testing, as this test is advocated in many clinical guidelines. Accordingly, the sole use of monofilament testing to diagnose peripheral neuropathy is not recommended.

The aim of this systematic review was to evaluate monofilament testing with the 5.07/10-g monofilament as a diagnostic test for peripheral neuropathy of the feet of any cause. An effective diagnostic test requires an acceptable and well-established sensitivity and an acceptable specificity. Sensitivity in the included studies ranged from 41% to 93%, and specificity ranged from 68% to 100%. These wide ranges are possibly due to differences in application of the monofilament (number and site), interpretation of the monofilament test (definition of thresholds), and differences in study populations. A meta-analysis was not possible because of this clinical heterogeneity.

The researchers believe their identification of studies has been complete, as no language restriction were applied and a sensitive search was conducted. The study with the best characteristics (Lee et al) showed a possibly serious methodological flaw: it was unclear whether the interpretation of the monofilament test was influenced by knowledge of the results of the reference standard and vice versa. In addition, a study population of 37 patients is quite small.

Another problem is the lack of standardization of the monofilament test methods. Different methods are described varying from 1 testing site to 10 testing sites on 1 foot, and there is no evidence or consensus about the most appropriate threshold.

Researchers found various published reference standards for peripheral polyneuropathy, including clinical examination, vibration perception thresholds with biothesiometer/vibrameter/tuning forks, warm/cold detection, and nerve conduction studies. Researchers rejected clinical examination, vibration perception, and warm/cold detection as reference standards: the first 2 because of obvious limitations in sensitivity or specificity, and thermal sense detection because it tests small-fiber neuropathy, whereas applying a monofilament and light touch, such as vibration and nerve conduction, tests for large-fiber neuropathy. Only 4 studies assessed with QUADAS used nerve conduction as the reference standard, of which 3 were included in the final selection.

Researchers also rejected studies if the monofilament test was performed on patients who had visible ulcers. In patients with current visible ulcers, the interpretation of the monofilament test and the nerve conduction studies may be influenced by this knowledge (observer or reviewer bias).

Researchers concluded that despite the frequent use of the (Semmes-Weinstein) monofilament test, little can be said about the test accuracy for detecting neuropathy in feet that do not have visible ulcers, because diagnostic studies with adequate methodology are lacking. Further research on monofilament testing should focus on optimal standard test application procedures (number and sites) and on defining a reproducible threshold.

As this test is already widely used and advocated in many clinical guidelines, especially for diabetic patients, standardization of the method for the mono-filament test and studies to define the sensitivity of this method in clinical practice are important. Meanwhile, the sole use of a monofilament test to diagnose peripheral neuropathy is not recommended. The diagnosis of peripheral neuropathy can be made only after a careful clinical examination with more than 1 test, as recommended by the American Diabetes Association. Tests for this clinical examination are vibration perception (using a 128-Hz tuning fork), pressure sensation (using a 10-g monofilament at least at the distal halluces), ankle reflexes, and pinprick. When in doubt, a nerve conduction test might be necessary to establish a firm diagnosis.

Annals of Family Medicine. 2009;7(6):555-558