From the Guidelines
The filament test, also known as the Semmes-Weinstein monofilament test, is a simple clinical examination used to assess peripheral nerve function and detect loss of protective sensation, particularly in diabetic patients, as recommended by the most recent guidelines 1.
Overview of the Filament Test
The test involves touching the patient's skin with a thin nylon filament that bends at a specific force (usually 10 grams), and asking the patient to report when they feel the touch.
- The test is typically performed on specific sites on the feet, including the plantar surface of the great toe, the base of the first, third, and fifth metatarsals, and the heel.
- Inability to feel the filament at one or more sites indicates sensory neuropathy and increased risk for foot ulceration.
- This test is valuable because it identifies patients who have lost protective sensation and are therefore at higher risk for undetected injuries.
Mechanism and Importance
The filament works by applying a consistent, reproducible pressure that stimulates mechanoreceptors in the skin, which transmit signals through large myelinated nerve fibers.
- When these nerve fibers are damaged, as in diabetic neuropathy, the patient cannot detect this standardized pressure, indicating compromised sensory function.
- The test should be performed with at least one other assessment (pinprick, temperature or vibration sensation using a 128-Hz tuning fork, or ankle reflexes) to confirm the diagnosis of loss of protective sensation (LOPS) 1.
- Absent monofilament sensation suggests LOPS, while at least two normal tests (and no abnormal test) rules out LOPS.
From the Research
Filament Test Overview
- The filament test, also known as the monofilament test, is a diagnostic tool used to detect peripheral neuropathy, particularly in patients with diabetes mellitus 2, 3, 4.
- The test involves the use of a monofilament, typically a 5.07/10-g monofilament, to assess the sensitivity of the feet to pressure and vibration 2, 3.
Diagnostic Accuracy
- Studies have shown that the monofilament test has varying degrees of sensitivity and specificity in detecting peripheral neuropathy, ranging from 41% to 93% sensitivity and 68% to 100% specificity 2, 3, 4.
- A systematic review and meta-analysis found that the pooled sensitivity and specificity of monofilament tests for detecting diabetic peripheral neuropathy were 0.53 (95% CI 0.32 to 0.74) and 0.88 (95% CI 0.78 to 0.94), respectively 4.
Clinical Use
- Despite its limitations, the monofilament test is still widely used as a screening tool for diabetic peripheral neuropathy, particularly in primary care settings 5, 6.
- However, the test should not be used as the sole diagnostic tool, and its results should be interpreted in conjunction with other clinical findings and diagnostic tests 2, 4.
- A three-site test involving the plantar aspects of the great toe, the third metatarsal, and the fifth metatarsals is recommended to maximize the diagnostic value of the monofilament test 3.