Monofilament test for diagnosing diabetic polyneuropathy has high rate of false negatives

The 5.07/10-g monofilament test for diabetic polyneuropathy may not perform as well in some patients as in others, a recent study found.

Researchers in Norway evaluated the diagnostic accuracy of the 5.07/10 g Semmes-Weinstein monofilament test in patients referred to neurological outpatient clinics for polyneuropathy assessments. Data were collected between May 2017 and December 2022. The monofilament test was validated against the Toronto consensus on diagnosing diabetic neuropathies as the reference standard, with results stratified by age, sex, and presence of neuropathic pain. Logistic regression was used to determine whether disease severity, as determined by a combined nerve conduction Z-score, was a predictor of diagnostic accuracy. The study results were published by BMJ Open Diabetes Research & Care on Nov. 20.

Five hundred six patients were included in the study. Of these, 39% were women, 66% had confirmed DPN according to the reference standard, and 54% had neuropathic pain. Mean age was 55 years. The global sensitivity of the monofilament test was 0.60 (95% CI, 0.55 to 0.66); specificity was 0.82 (95% CI, 0.75 to 0.87); positive and negative predictive values were 0.86 (95% CI, 0.81 to 0.90) and 0.52 (95% CI, 0.46 to 0.58), respectively; and positive and negative likelihood ratios were 3.28 (95% CI, 2.37 to 4.53) and 0.49 (95% CI, 0.42 to 0.57), respectively. The test was found to be less sensitive in women (0.43) and less specific in patients with neuropathic pain (0.56). It also performed worse in patients who were 50 years of age and older versus younger patients. Disease severity based on nerve conduction score had no effect on diagnostic accuracy (odds ratio, 1.15; 95% CI, 0.95 to 1.40).

The researchers noted that their measure of disease severity did not necessarily reflect clinical severity and that the monofilament test was validated in tertiary rather than primary care, among other limitations. They concluded that the 5.07/10-g Semmes-Weinstein monofilament test did not perform well in patients with diabetes referred for polyneuropathy assessments. “The clinical value of the [test] for this population is limited: the high rate of false negatives with almost half of DPN [diabetic peripheral neuropathy] cases missed, and the subsequent low [negative predictive value] renders negative results meaningless,” the authors wrote. In addition, the test performed particularly poorly in women and in patients with neuropathic pain and its diagnostic accuracy did not improve in patients with more severe disease. “We do not recommend the use of the 5.07/10 g monofilament in the evaluation of patients with diabetes referred to polyneuropathy assessments,” the authors wrote.

An accompanying editorial said that the study results were not surprising and were consistent with previous research. “It may be time to challenge this failed strategy, given that the rate of DFU [diabetic foot ulceration] and amputation as well as re-amputation is increasing, and the vast majority of patients with DPN remain undiagnosed,” the editorial said. “A systematic screening strategy is required for the diagnosis of early DPN and risk factor interventions targeting obesity, physical inactivity, hyperglycemia, hypertension and hyperlipidemia to limit progression to more advanced DPN and DFU.” Physicians must perform a comprehensive neurological examination, including pin prick and thermal perception to assess small fiber damage, rather than relying on the 10-g monofilament test, the editorialist recommended.