There are many diseases and health conditions that lead to or increase risk of loss of protective sensation (LOPS) of the foot. Some of the most common are diabetes, spina bifida, Hansen's Disease, systemic lupus erythematosus, human immunodeficiency virus infection, AIDS, AIDS-related complexity, cancer, vitamin B deficiency, multiple sclerosis, uremia, vascular disease, and Charcot-Marie-Tooth disease.1–5 Loss of protective sensation will raise the risk for undetected injury that can ultimately result in ulceration, infection, and amputation.1–4,6–8 Impaired sensation can also compromise gait and balance, increasing the risk for falls.7,8 Advanced age and altered foot biomechanics increase this risk when combined with pathologies that can precipitate LOPS.7–10
Prevention of the morbidity caused by LOPS can be facilitated by effective screening and clinical examination sensitive enough to detect changes in a persons' protective sensation. The 5.07, 10-g monofilament is used as a screening tool to determine whether further evaluation of protective sensation in the feet is warranted.1–3,11–13 This screening strategy is time-efficient, easy-to-use, low-cost, and it has demonstrated predictive ability and reliability.1–3,14–16 To screen for the presence or absence of protective sensation in the diabetic foot, the American Diabetes Association has recommended performing a 10-g monofilament test at the plantar hallux and metatarsal heads, and 1 additional clinical test (eg, vibration testing using a 128-Hz tuning fork, tests of pinprick sensation, ankle reflex assessment, and testing vibration perception threshold with a biothesiometer).1,2 However, even this recommendation is not without dispute in the literature; “The combination of two simple tests (eg, the 10-g SWME and vibration testing by the on-off method) does not add value to each individual screening test.”15
The literature is not specific on how many test sites on the foot are necessary when using the 10-g monofilament to establish LOPS.4,7–10,13–24 Loss of protective foot sensation in persons with different pathologies may not be uniform, requiring monofilament testing of multiple sites to assess specific dermatomes and peripheral nerve distributions.
While the current literature reports good reliability of the monofilament to detect neuropathy when compared to other tests of foot sensation,9–11,13,14,16,23–28 we found only 1 report that reported the intraclass correlation coefficient (ICC) statistic.25 No other reports offer any information regarding the responsiveness of a multisite examination.21,25,26 Also, reports often do not individually consider protective sensation and frequently group all monofilament forces together when reporting reliability.7,8,25,28
The suggestion has been made that inability to sense the 10-g monofilament at even 1 site would direct the clinician to consider that the person has impairment of protective sensation.7,14,22 However, the increased health care cost of inappropriate diagnosis of LOPS, as well as the imperfect reliability associated with monofilament testing, suggest that more information is needed to guide the clinician.12 The authors sought to establish the reliability and responsiveness, via minimal detectable change (MDC), for an 18-site (9 per foot) examination of protective sensation in the feet using a 10-g monofilament.
After obtaining approval from the institutional review board, a convenience sample of 28 participants (mean age = 61.12 years, SD = 9.05; 20 women and 8 men) was recruited from a diabetes fair and via flyer to participate. Among the participants, 6 had a diagnosis of diabetes and 3 had a diagnosis of peripheral neuropathy. To be included in the study, participants had to be 50 years or older, have no cognitive impairment, be English speaking, and be able to lie supine for up to 15 minutes. Potential participants were excluded from the study if they had any skin ulceration of the lower extremity, any active malignancy, and any prior joint replacement in the lower extremity.
Three testers reviewed the testing protocol that came from the manufacturer of the monofilament (North Coast Medical, Inc, 18305 Sutter Boulevard Morgan Hill, California) and practiced briefly on each other until they felt prepared to begin testing participants (5–10 practice trials). Testing procedures followed those outlined in the manufacturers' instructions and are similar to other procedures described in previous studies: the participant was positioned in supine, the testing procedure was demonstrated on the participants' hand or forearm along with a brief description of the test.11,19,22,25 Participants were instructed to alert the tester whenever they felt the monofilament on any of the test sites by saying “yes” and to remain silent if no sensation was experienced. If participants said, “yes” when the monofilament was not in contact with the skin, nothing was recorded. However, the sites tested just before and after this verbalization were repeated. Participants kept their eyes closed throughout the testing. A single individual provided instruction to the participant for all 3 testers, who did not verbally interact with the participant. Each tester applied the filament to the target sites on the plantar surface in random order. Order of testers for each participant was also random. Because their eyes were closed, participants were blinded to each tester. Testers were blinded from the results of their fellow testers, as well as their previous results. Participants were tested twice, in sessions no more than 1 week apart. In each session, the filament was applied to 18 sites (9 on each foot) representing dermatomes and peripheral nerve distributions (Figures 1 and 2).
To determine the intratester reliability of the 18-site 10-g monofilament test, the average intratester reliability value from model 3 of the ICC was computed using SPSS, version 17.0 (SPSS Inc, Chicago, Illinois).29 To determine intertester reliability for the 3 testers, model 2 of the ICC was used.29 Interpretation of the ICC values for this study follows commonly used grouping, where an ICC value of less than 0.40 indicates poor reproducibility, ICC values in the range 0.40 to 0.75 indicate fair to good reproducibility, and an ICC value of greater than 0.75 shows excellent reproducibility.30 Using the reliability value from the intertester ICC, the Standard Error of Measurement (SEM) was calculated with the following formula:29
where rxx = test-retest reliability.
For an individual, MDC at a 95% confidence level (MDC95) can be calculated by multiplying the SEM by 1.96 (representing 95% of the area under a normal distribution curve) and 1.41 (the square root of 2) to control for possible error when calculating the coefficient from 2 data sets (test and retest in this example):31
Individual MDC95 is typically used as a benchmark for change in individual clients or participants. Group MDC95 is frequently used by researchers or clinicians to detect a statistically significant change in the mean score of a group. To calculate the MDC95 for a group, MDC95 for the individual is divided by the square root of the number (n) of people in the group:31
The intratester ICC (3, 1) and MDC95 were calculated for each tester and then averaged yielding the following: ICC = 0.76 (95% CI: 0.68–0.84), Individual MDC95 = 2.83, Group MDC95 = 0.53. The ICC (2,1) for intertester data was calculated and yielded the following: ICC = 0.78 (95% CI: 0.64–0.88), Individual MDC95 = 2.81, Group MDC95 = 0.53.
Results from the present study support other studies indicating that the 10-g monofilament is a reliable tool. Also, responsiveness of the tool using MDC95 in an 18-site examination was assessed. A 3-site change in protective sensation was determined to be the MDC at a 95% confidence interval on an individual level. Therefore, when performing an 18-site examination of protective sensation using the 10-g monofilament, a change in 3 or more sites would allow the clinician to be 95% confident an actual change in protective sensation had occurred. This allows for more accurate and confident referral, client education, and disease progression or regression measurement.
A change of at least 0.53 sites was determined to be the group MDC95. Therefore, if a group of patients has realized a mean change of 0.53 sites on the 18-site monofilament examination, then researchers or health care providers could confidently conclude that group of patients had a statistically significant change in their protective sensation. While the group MDC is not typically used by health care providers, it can be used to determine whether the mean score of a group of participants with similar characteristics has changed significantly from a previous measurement. In the case of an 18-site examination for LOPS, it could be used to determine whether a group of similar patients (eg, (50 years old with diagnosed DM) in an outpatient clinic were experiencing a significant change in their protective sensation from 1 year to the next by taking the difference in the means between the 2 years.
A limitation of this study may be that only 1 touch of the monofilament per test site was performed during testing. While this was according to the manufacturers' instructions, other researchers have used up to 10 touches per test site before a determination was made on the sensation of that site.9,13–16,19,25,32,33 The authors recommend that each site be tested 3 times and the best 2 of the 3 responses be used to score the site as described by other researchers.19 The participants in this study demonstrated experimenter expectancy33 by occasionally saying “yes,” when the monofilament was not in contact with the skin. In future studies, the authors recommend that participants be asked to point to the foot location on a diagram where they felt the monofilament.
Also, it may have been difficult for multiple testers to consistently place the monofilament in exactly the same test location. In future studies, drawing circles or dots around or on site locations on the participants' feet before testing may help to reduce or eliminate intertester inaccuracy when placing the monofilament.
An 18-site 10-g monofilament examination demonstrated good reliability and had an individual MDC95 of 3 sites for the participant pool in this study. By repeating the examination over time and comparing the results, clinicians can be confident that a change in protective sensation at 3 or more sites on the 18-site examination represents actual improvement or decline in the protective sensation of their individual clients.
The authors thank Merrill Landers, PT, DPT, for his assistance with the statistical analysis of this manuscript.
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