Evaluation of skin susceptibility to irritancy by routine patch testing with sodium lauryl sulfate

ARTICLE

There are certain reasons why irritant patch testing is needed. One major reason is to determine the individual skin susceptibility [1-4]. Although it has been shown that the skin reaction to one irritant cannot predict the response to another [5, 6], the SLS patch test has become a widely used irritant assay [7], as SLS (because of its characteristics) and seems to be an appropriate test irritant, particularly for the risk group of wet workers [8].

Mostly, irritant patch testing has been performed on the forearm. The predominant method for evaluation, especially of subclinical skin reactions, is the measurement of transepidermal water loss (TEWL) [9]. Often, the skin susceptibility of the back is of major interest, particularly when many test areas in an allergic patch test shows irritant reactions. In these cases, an irritant patch test should be performed on the back also. The comparison between this SLS patch test on the back, evaluated visually, compared to the standard SLS patch test on the forearm, evaluated visually and with TEWL measurement is the aim of the study. Moreover, we wanted to define a SLS concentration with the best specificity and sensitivity for visual evaluation compared to the standard method of TEWL-measurement.

Patients and method

Study population

One hundred and sixty-nine volunteers (95 women and 74 men), aged between 18 and 60 years, participated in this multicenter study. The participants were recruited from the out-patient and in-patient clinic of the Department of Dermatology, Philipp University of Marburg and University Witten/Herdecke. Informed consent was obtained from all participants, and the study was approved by the ethical committee of the University of Marburg.

Test procedure

Epicutaneous patch testing was performed with 60 µl SLS (SLS Sigma, 99% purity) in Large Finn Chambers^® (inner diameter 1.2 cm, Epitest Ltd., Hyrlä, Finnland) for 48 hrs on clinically unaffected skin simultaneously at the flexor side of the forearm and on the back. On each test site, aqueous SLS at following concentrations was tested: 0.125, 0.25, 0.5 and 1.0%. The order of application was chosen randomly. Evaluation was performed 30 min and 24 hrs after removing of the patch.

On both test sites, skin response was visually scored following the suggestion of the Standardization Group of the European Society of Contact Dermatitis [9]: 1: mild erythema, 2: moderate erythema and infiltration, 3: severe erythema and infiltration, 4: with necrosis.

On the forearm, TEWL measurement was performed additionally using the TEWAMETER TM210 (Fa. Courage & Khazaka, Cologne, Germany) and following the guidelines of SLS exposure tests by the Standardization Group of the European Society of Contact Dermatitis [10]. Before measurement, volunteers had rested 1/2 hr at a room temperature between 20-22° C.

The following points were investigated:

1. Difference in skin responses between the different test areas.
2. Correlation between TEWL values of the forearm and visual scores of the forearm and back.
3. The SLS concentration which might yield the best test sensitivity and specificity.

Statistics

Data were calculated with SPSS for Windows. After calculation with the Kolmogorov-Smirnov-Test, the visual scores and TEWL values were shown as median. Differences between test sites were calculated using Wilcoxon-test, and correlation analyses were calculated using Pearson-correlation-coefficient. Calculation of test sensitivity (Sens) and specificity (Spec) were calculated as follows:

Spec = RN : (RN+ FP)
Sens = RP : (RP + FN)
(RN = right negative, RP = right positive, FN = false negative, FP = false positive)

A cut off TEWL level was defined as a value higher than 75% of all measured TEWL values. Values higher than the cut off level were defined as pathological.

The specificity is defined as the rate of correct negative test results (visual score ¾ 1) compared to the sum of normal TEWL values. The sensitivity is defined as the rate of correct positive test results (visual score > 1) compared to the sum of pathologic TEWL values.

Results

Descriptive statistics

As the Kolmogorov-Smirnov-Test showed no symmetrical distribution, descriptive statistics were performed and are listed with median values in Table I.

Difference in skin responses
between the different test areas

There was a high correlation (p < 0.01) between SLS concentration and skin reaction at 48 and 72 hrs evaluated by both methods (TEWL and visual scoring). In addition, the visual scores of the forearm correlated significantly with those of the back at 48 and 72 hrs (p < 0.01). A comparison of the visual scores between the forearm and the back resulted in pronounced reaction to SLS at the forearm (Table I).

Correlation between TEWL values of the forearm
and visual scores of the forearm and back

On the forearm, the TEWL values correlated well with visual scores at 72 hrs (p < 0.01). At 48 hrs the correlation was solely significant at 0.5% SLS (p < 0.05) and 1.0% (p < 0.01). There was, however, no significant correlation between TEWL values of the forearm and the visual scores of the back (Table II).

Best test sensitivity and specificity

For a calculation of the test sensitivity and specificity, a reference method needs to be defined. As we were particularly interested in the reliability of the visual evaluation, the TEWL measurement of the forearm provides herein a reference method. A cut off level was defined as a TEWL value higher than 75% of all measured values at one concentration as listed in Table III.

The calculated rates of test specificity and sensitivity with a cut-off value of 75% are shown in Table IV. The most useful response was yielded by 0.5% SLS on the forearm.

Table V shows the cumulative percentage of volunteers who had a visual score of ¾ 1.

Discussion

The SLS patch test represents a useful assay for determining the individual skin sensitivity [8, 11, 12]. In the near future, SLS patch test will perhaps be used as a routine irritant patch test in addition to an allergic patch test to other contact allergens. Although TEWL measurement is said to be the most accurate evaluation method for SLS patch test [13], visual scoring will certainly be the most appropriate one for a routine test. Moreover, it would be practical if the test could be performed in the same manner as a routine allergic patch test: a 48 hr SLS application on the back. We evaluated the SLS patch test under such conditions in this study.

A tendency to higher visual scores at the forearm has been obtained, when compared to those of the back. The lack of significant difference at lower SLS concentrations may be caused by numerous negative skin reactions at both test sites. These findings confirm previous studies, implying the forearm is more susceptible to irritants than the back [14, 15]. When conducting SLS patch tests on the back, the SLS concentration should be at least as high as that of the forearm in order to yield comparable results.

Interestingly, TEWL values were higher at 48 hrs than at 72 hrs. By contrast, the visual score was higher at 72 hrs than at 48 hrs. Pronounced reactions at 72 hrs were, indeed, reported in other studies, too [12, 16]. The kinetics of TEWL in our study could be explained by the fact that TEWL was measured 30 min after removing the patch. Agner et al. [17] found stable TEWL values only 60 min after patch removal. This measurement point of time might give accurate data, but, since it is time consuming, it is not practical for routine testing. So, if a resting time of 60 min after patch removal can not be performed at 48 hrs, the measurement should be done the next day (72 hrs).

Because the correlation between the skin reaction and SLS concentration can be detected by TEWL measurements and visual scoring as well, both methods are capable of determining the skin response to SLS sufficiently. This confirms the previous findings of other investigators [8, 18-21].

One of the main purposes of the study was to compare the test outcomes between the forearm and the back. There was, indeed, a strong correlation in visual scores between both test areas. This underlines once again that skin susceptibility, at least to SLS, is an individual factor which may rather apply to the entire skin [19, 22].

The correlation between TEWL values and visual scores was convincing at the forearm. But, there was no significant correlation between TEWL values of the forearm and visual scores on the back. Two variables have to be taken into consideration in this comparison: the methods of evaluation and the test area. If only one variable has been changed (e.g. TEWL vs. visual scores on the same area, or forearm vs. back with the same assessment) the correlation was significant. When both variables are involved (TEWL forearm vs. visual scoring back) influencing factors seemed to increase leading to a non-significant correlation. Hence, testing on the forearm should be suitable for evaluation of skin susceptibility of the hands or forearm, while testing on the back should be performed for evaluation of skin susceptibility on the back (for example in addition to an allergic patch test to other contact allergens).

When determining the reliability of a skin test, its sensitivity and specificity are relevant [23]. Taking TEWL measurement as a reference method with a cut-off level of 75% (i.e. 75% of all tested volunteers had lower TEWL values) for a pathological test result, we were able to calculate the sensitivity and specificity rates of the test for each SLS concentration and time point of evaluation. At low SLS concentrations mostly a high specificity was found, implying that most of the visual scores are (correctly) negative. However, the sensitivity of the test is very low, suggesting most of the visual scores were erroneously negative, even when TEWL was higher than average. On the other hand, the test sensitivity was increasingly better when the specificity worsened, since the visual score is somewhat positive, even when TEWL is normal. Based on our data, the best combination of specificity and sensitivity of SLS patch test has been found with 0.5% SLS at 72 hrs on the forearm with a specificity of 0.69 (i.e. 69% of all volunteers with normal TEWL also showed no visible reaction) and a sensitivity rate of 0.97 (i.e. 97% of all volunteers with high TEWL values also showed visible skin responses). A calculation of specificity and sensitivity of SLS patch test on the back can not be performed, as there was no correlation between TEWL on the forearm and visual scoring on the back.

Since the aim of an irritation test is to identify subjects with an increased skin susceptibility, a 48 hrs patch test with SLS 0.5% on the forearm, evaluated after 72 hrs seems to be appropriate (cut off level: 75%, rate of sensitivity: 0.97). To achieve a similar cut off level on the back, the 0.5% SLS test (with evaluation after 72 hrs) should be used too (cut off level: 0.73%, Table V). This was backed up by the correlation in the visual scores between the back and the forearm.

Our data provides the following cut off values of 0.5% SLS test evaluated after 72 hrs :

- TEWL value should be ¾ 31.6 g/m²/hr on the forearm (25% showed higher TEWL values);
- visual score should be ¾ 1 (higher values were obtained in 40% on the forearm and 27% on the back).

Although a contact allergy to SLS can not be excluded, it is not likely. The rare description of a contact allergy to SLS is not reliable, because the patch testing which was performed to prove this suspicion was performed with high SLS concentrations [24], where many patients do still react with clear irritant reactions [9]. When a contact allergy to SLS is assumed, we suggest an allergic patch testing with at least two SLS-concentrations much lower than the one used for irritant testing, e.g. 0.05 and 0.01%. In this test protocol, a strong (not concentration depending) reaction to SLS could be an indicator for a SLS allergy. The morphology and time course of the test reactions may prove helpful to decide whether the reaction is an allergic or an irritant one [16].

Taken together, the forearm provides a test area with a high skin susceptibility to SLS. Visual scoring of skin responses in SLS patch test seems to be sufficient for a routine assessment. Reliable results on the forearm have been obtained with a 48 hrs application of SLS 0.5% evaluated after 72 hrs. The average TEWL value seems to be ¾ 31.6 g/m² hr, the normal visual score ¾ 1. A routine irritant patch test with SLS 0.5% on the back which is evaluated visually could give a first estimation of skin susceptibility. Assessment of test reactions with the time-consuming TEWL measurement is the gold standard for studying experimental irritant skin reactions, particularly when various SLS concentrations are used. *

Article accepted on 29/3/01

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