Monofilament Test Sensitivity Questioned for DPN Screening

Two studies presented in June at the annual meeting of the American Diabetes Association underscored the shortcomings of monofilament testing alone as a screen for diabetic peripheral neuropathy (DPN). The sensitivity of the monofilament has been questioned, and at least one study¹ has shown that the quality of the device differs among manufacturers and degrades with repeated use, sometimes within a relatively short time.

Monofilament testing is, however, by far the most commonly used technique to screen patients with diabetes for peripheral neuropathy. Because it is inexpensive and relatively easy to administer, this test is particularly well suited for underserved populations. However, because monofilament tests consistently fall short compared with alternative screening methods in clinical studies, practitioners are increasingly opting to combine monofilament testing with at least one other screening test for better accuracy.

Sixteen percent of 166 patients with diabetes (mean duration of 9 years) tested positive for neuropathy with a conventional 5.07-gauge (10-g) Semmes-Weinstein monofilament in a study conducted by researchers from the International Diabetes Center in Minneapolis. However, 31% of patients met the team's clinical definition of DPN, which required a score of 2 or higher on the Michigan Neuropathy Screening Instrument (MNSI) test and either abnormal vibration threshold levels or abnormal nerve conduction. Sixty-three percent of patients exhibited symptoms of diabetic neuropathy, as indicated by a positive Neuropathy Total Symptom Score.

The Minneapolis study, led by Mary L. Johnson, RN, director of clinical research at the International Diabetes Center, also found that duration of diabetes was associated with the prevalence of microvascular complications (MVCs), including retinopathy and nephropathy, as well as peripheral neuropathy. In the 87 patients with no MVCs, the mean disease duration was 6.9 years, compared with 9.9 years in those with 1 MVC and 11.5 years in those with 2 MVCs. Peripheral neuropathy was the most commonly observed MVC.

The 5.07 monofilament fared no better in a Spanish study of 305 patients with diabetes. In this study, the microfilament test was compared with the MNSI and 2 vibration threshold measures, the quantitative tuning fork and neurothesiometer tests.

Juan Ybarra, MD, and Jose M. Pou, MD, PhD, of the Hospital Sant Pau in Barcelona and Jeronimo Jurado of the Catalan Institute of Health in Olot found that the prevalence of DPN was 23.17%; however, none of the 4 tests came close to diagnosing that many cases. The quantitative tuning fork test diagnosed the highest percentage of cases (16.34%), while the lowest percentage (9.77%) was diagnosed by the monofilament test. All 4 tests demonstrated high levels of specificity but low levels of sensitivity.

As a general rule, screening tests tend to err on the side of high specificity at the expense of sensitivity to minimize the number of false-positive results and unnecessary follow-up tests. Even so, the findings from Minneapolis and from Spain support the increasingly popular belief that an effective screening tool for neuropathy should have higher levels of sensitivity than those associated with the monofilament test. Research from the University of East London, presented last summer at the World Congress of Podiatry in Boston, suggested that the stimulus delivered by 5.07 monofilaments may not be subtle enough to diagnose impaired sensation accurately.

In addition, a study published in Diabetes Care in July 2000¹ suggested that 5.07 monofilaments may not necessarily deliver 10-g of force. Matthew J. Young, MD, a consultant diabetologist at the Royal Infirmary in Edinburgh, and colleagues found that the ability of monofilaments to buckle at 10-g of force varied considerably among manufacturers. A total of 160 new 5.07 monofilaments from 4 different manufacturers were tested. The results indicated that the monofilaments buckled at mean forces ranging from 8- to 10-g. The rate at which monofilaments buckled between 9- and 11-g of force was 100% for 2 of the brands at the beginning of the study, but the rate for these 2 brands had slipped to 80% after 100 compressions and to 50% after 200 compressions. *

REFERENCE

1. Booth J, Young MJ. Differences in the performance of commercially available 10-g monofilaments. Diabetes Care. 2000;23:984-988.