Feasibility and Utility of the Fric Test in Symptomatic Dermographism: A Pilot Study

Sir,

Symptomatic dermographism is the most common type of physical urticaria and accounts for almost 7%–10% of all urticarias.[1] Dermographism is characterized by the development of a wheal and flare reaction in response to a stroking or rubbing stimulus, usually occurring within minutes of the inciting stimulus. Symptomatic dermographism can be differentiated from simple dermographism by the presence of itching and a generally lower urticariogenic threshold.[1]

Scientifically, a positive diagnosis is made if a wheal appears on the skin within 10 minutes in response to a moderate stroking stimulus of <36 g/sq.mm.[2] A provocation threshold is defined as the lowest level of shear force needed to provoke a positive wheal response.

Conventionally, diagnosis was made by stroking the skin with a wooden tongue blade or a closed ballpoint pen with an empty refill.[3,4] The intensity of the stroke produced by the ballpoint pen can be highly variable. Hence, a properly validated scientific method of eliciting dermographism has been developed recently. This instrument is known as the Fric test or the friction test dermographometer (Charité Hospital, Berlin, Germany).[5] It is a flat, rectangular plastic template with four smooth plastic tips of varying lengths (2.5, 3.0, 3.5, and 4.0 mm), set 2 cm apart [Figure 1]. Each tip is about 3 mm in diameter with a slightly rounded end.

Figure 1.

Fric test instrument with four rounded pins and a comb-like design.

We conducted a pilot study to assess the provocation thresholds generated by the Fric test in symptomatic dermographism when compared to the ballpoint pen test. We selected 30 newly diagnosed cases of symptomatic dermographism coming to our OPD and 30 controls with no active pruritic skin disease [Table 1]. The cases and controls were comparable for mean age and male: female ratio.

Table 1.

Findings recorded in our study

In all the patients, on the left arm, a firm stroke of ~10 cm was given with the help of a ballpoint pen. On the right arm, the Fric dermographometer was used. The instrument was held perpendicular to the forearm, and applying just enough pressure so that the tips are almost invisible, it was drawn across in a single firm stroke [Figure 2]. The extent of whealing was measured on both the sides after 10 min, excluding any wheal that developed thereafter. Development of wheal along with pruritus at the site of largest test pin was considered a positive response. The significant wheal diameter was defined as 3 mm.[6]

Figure 2.

Using the Fric test dermographometer.

A positive response with all the four pins, including the 2.5-mm pin which would exert the lowest stroking stimulus, suggested severe dermographism. A wheal with only the largest pin (4 mm) was considered as a positive response, but with milder grading.

After the study, we can say that the Fric test was more likely to be positive in patients with symptomatic dermographism as compared to the ballpoint pen test [Table 2]. However, this difference did not reach statistical significance (P > 0.05).

Table 2.

Comparison of Fric test and ball point pen test

Clinically, a need was felt for a reproducible stimulus which can elicit and assess dermographism. The Fric test has been designed to serve this purpose. The comb-like design of the Fric test allows clinicians to obtain a graded and reproducible wheal response. It enables the determination of provocation thresholds. The Fric test can also be used to gauge the efficacy of therapy. However, these findings will have to be validated by larger clinical studies.

A study was done comparing the Fric test instrument with a single-tip dermographic tester. The results showed that a graded and reproducible provocation of wheals could be obtained using the Fric test instrument in patients with symptomatic dermographism.[5]

The authors recommend that the Fric test can be performed in patients who are off anti-histamines for duration of at least 3 days (48 to 72 h), which would be an acceptable time for assessment of response in most symptomatic patients. Longer durations off anti-histaminics would not be reasonable in very symptomatic patients. However, longer durations may be needed with drugs such as doxepin.

The older version of the Fric test instrument, Fric Test 3.0,[6] had six pins. However, it has been further developed into a simpler four-pin instrument which is currently in use and has better sensitivity and specificity.[6]

A Fric Test dermographometer should be cleaned after each use with an ordinary alcoholic disinfection solution. It may be cleaned with water, soap and a brush. It must not be sterilized by heat or any other sterilization method.

As the test is non-invasive when performed correctly, ethics committee approval would usually not be required before performing this simple test in the Dermatology OPD.

The Fric Test dermographometer is currently not available in India. It is available in various other countries including Germany, where it was first designed.