Family Practice Vol. 16, No. 6, 611-615
© Oxford University Press 1999
Diagnosing dermatomycosis in general practice
Department of General Practice, University of Leuven and
a Department of Dermatopathology, CHU Sart-Tilman University of Liège, Belgium.
Dr D Lousbergh, Academisch Centrum voor Huisartsgeneeskunde, Kapucijnenvoer 33, Blok J, B-3000 Leuven, Belgium.
Lousbergh D, Buntinx F and Piérard G. Diagnosing dermatomycosis in general practice. Family Practice 1999; 16: 611–615.
Received 22 February 1999; Revised 21 May 1999; Accepted 22 June 1999.
Abstract
Background. Diagnosing dermatomycosis from a clinical image is not always easy. Microscopy of a potassium hydroxide preparation (KOH-test) and culturing are seldomly used in general practice. Cyanoacrylate surface skin scraping (CSSS) is a new diagnostic tool that may be useful and simple.
Objectives. We aimed to investigate the diagnostic value of signs and symptoms, the KOH-test and the CSSS, in patients with erythematosquamous skin lesions, using the culture as the gold standard. Our goal is to formulate an optimal algorithm for the diagnosis of mycosis, based on one or more of these tests and including both optimal accuracy and costs.
Methods. Scales from 148 consecutive general practice patients were tested using a KOH-test, CSSS and culture. Clinical data were collected using a questionnaire.
Results. Twenty-six (18%) positive fungal cultures were identified. The sensitivity of the clinical diagnosis was 81% and its specificity 45%; for the KOH-test, these figures were 12 and 93% respectively; and for the CSSS, 62 and 88%, respectively. The positive predictive value of the clinical diagnosis was 24% and the negative predictive value 92%; for the KOH-test these figures were 25 and 83%, respectively, and for the CSSS, 52 and 92%, respectively. Determining CSSS in all patients proved to be the most accurate policy (accuracy = 83%). The likelihood ratio of CSSS in all patients was 5.17 for a positive test result and 0.43 for a negative test result. An approach in which CSSS is obtained in only those patients whom the physician considers by clinical examination to have dermatomycosis, with no testing in other patients, results in positive and negative likelihood ratios of 4.69 and 0.56, respectively. Such a policy would result in an overall sensitivity of 50%, a specificity of 89%, a positive predictive value of 50% and a negative predictive value of 89%.
Discussion. The clinical picture of dermatomycosis is not very reliable. The combination of a clinical judgement if this is negative and an additional CSSS in the case of a positive clinical judgement provides us with the best cost–benefit ratio, if both diagnostic accuracy and logistic considerations are taken into consideration.
Keywords. Cyanoacrylate surface skin scraping, dermatomycosis, potassium hydroxide test.
Introduction
In spite of its high prevalence (5.4/1000),1,2 dermatomycosis is frequently misdiagnosed in general practice.3,4 The diagnostic value of signs and symptoms is doubtful; using the potassium hydroxide test (KOH) could confirm the diagnosis, but is often passed over.5–12
Classical culture is the generally accepted gold standard, but has no impact on bedside diagnosis.6,13–15 The cyanoacrylate surface skin scraping (CSSS) is a strip biopsy where a layer of horn is removed together with the adhesive.16–28
A correct diagnosis of erythematosquamous skin lesions is important, as cheap and safe treatment is available. Inappropiate treatment with combinations of corticosteroids and antimycotics should be avoided. The possible side-effects of oral treatment have to be considered, together with its cost.29–32
Therefore we started a study to examine the diagnostic value of the signs and symptoms, using the KOH-test and the CSSS method, and using the culture as a reference test, in patients with an erythematosquamous skin lesion. We intended to formulate an optimal algorithm for the diagnosis of mycosis in these patients, including both diagnostic accuracy and cost in the calculations.
Methods
The study was carried out in 27 general practices in Flanders. Consecutive patients with an erythematosquamous lesion of the glabrous skin were eligible, with exclusion of lesions already treated with topical or systemic antifungal therapy.
Patient characteristics, local signs and symptoms were identified. The GP registered his diagnosis as either a mycosis or another disorder, as well as his treatment. Scales were collected from the border of the lesion, for microscopic testing with KOH and culture.
A CSSS was made from the same site. A drop of cyanoacrylate adhesive (Loctite glue®) was deposited on a sheet of terephthalate polyethylene (Melinex O®, ICI) and pressed firmly onto the degreased lesion. After about 30 seconds, a sheet of uniform thickness of stratum corneum was removed. Within 24 hours it was sent to the laboratory of dermatopathology of the University of Liège.
Skin scrapings were inoculated onto Mycoline® slides. Examination followed after incubation at 28–30°C for 4 weeks. Microscopy was performed with KOH 10%, using pen ink for staining. The CSSS was coloured with polychrome multiple stain. All slides were read by the same experienced technologist.
Results of the clinical examination, the KOH preparation and the CSSS were compared against culture results. A decision tree was produced, comparing all possible combinations signs and symptoms with or without KOH-test and/or CSSS. The accuracy of each resulting 2 x 2 table was used to decide which was the best algorithm. For the combinations with the highest accuracy, likelihood ratios for a positive (LR+) and for a negative test result (LR–) were calculated as a confirmation. Financial costs of the diagnosis and initial treatment results from one of the algorithms were based on £17.60 for a CSSS, and £3.70 for local antifungal treatment. Patients diagnosed as without mycosis are supposed to be treated with local corticoids at £3.40. Prices related to one package of a generally used product in Belgium. To make comparisons possible, all patients on antifungal treatment are considered to be treated with local drugs. Financial costs are balanced against the sum of false positive and false negative diagnoses.
Results
Thirty-four GPs participated in the study. A total of 170 specimens from the same number of different patients were sent to the laboratory; 148 were admitted for analysis.
The most common diagnoses, apart from dermatomycosis were Pytiriasis rosea (10%) and Nummular dermatitis (9%). There were 40 positive cultures. After eliminating 14 cultures, considered as a contamination or as non-pathogenic, 26 positive fungal cultures were identified. The most frequently isolated dermatophyte was Trichophyton rubrum (n = 9).
In 21 cases with a positive culture, the clinical diagnosis of the GP was correct (sensitivity = 81%); the KOH correctly identified three cases (12%) and the CSSS 16 cases (62%).
Out of the 122 negative specimens, 67 (55%) were falsely diagnosed as a mycosis based on the clinical examination, 9 (7%) were read as false KOH positive and 15 (12%) as false CSSS positive.
A lesion diagnosed by the GP as a dermatomycosis resulted in a positive culture in 24%. The negative predictive value of the clinical diagnosis was 92%. A specimen read as KOH positive resulted in a positive culture in 25%. Its negative predictive value was 83%.
The CSSS showed a positive predictive value of 52% and a negative predictive value of 92% (Table 1
).
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The sensitivity of individual signs and symptoms ranged between 4% (yellow colour) and 77% (minimal scaling), and the specificity between 20% (minimal scaling) and 96% (brown colour) (Table 2
).
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After calculating the accuracy of each possible combination of test results (Table 3
), determining CSSS in all patients, irrespective of the clinical picture, proved to be the most effective policy (accuracy = 83%). In patients with a positive clinical diagnosis, the accuracy of the CSSS was considerably higher compared with the next best (KOH-test with or without CSSS). In patients with a negative clinical diagnosis, however, the difference between all possible policies was minimal. In these patients not implementing any additional test resulted in only one additional false result. For the whole group, the combination of accepting a negative clinical judgement, and performing a CSSS in patients with a clinical picture of dermatomycosis only, resulted in an accuracy that was almost identical to performing a CSSS on all patients (82%).
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The likelihood ratio of CSSS on all patients was 5.17 (95% CI = 2.85–8.75) for a positive test result and 0.43 (95% CI = 0.27–0.74) for a negative test result. When applying CSSS to patients with a positive clinical picture, with no additional test in patients with a negative clinical picture, these values were 4.69 (95% CI = 2.47–8.91) and 0.56 (95% CI = 0.38–0.83), respectively. Such policy would result in an overall sensitivity of 50%, a specificity of 89%, a positive predictive value of 50% and a negative predictive value of 89%.
The total financial cost per patient for the actual treatment, for the prescription of a CSSS test in all patients and for the prescription of a CSSS test in patients with a clinical diagnosis of mycosis only, followed by the consequent local treatment, are estimated at £3.60, £21.00 and £14.00, respectively.
Discussion
The prevalence of dermatomycosis in general practice is high1,2 and commonly misdiagnosed by GPs.3,4 Elements of the clinical picture are not very reliable. Although its sensitivity (81%) is highly acceptable, more than three out of four clinical diagnoses could not be confirmed by culture. According to our results, 71% of the oral treatment and 76% of the topical treatment were inappropiate.
The KOH test results in significant additional diagnostic value if the GP suspects a positive diagnosis based on the clinical picture. The optimal diagnostic value was reached with a CSSS test performed on all patients (LR+ = 5.17, LR– = 0.43). However, compared with a clinical diagnosis only, the only benefit of this test is for patients with a clinical picture suggesting dermatomycosis.
Considering the low extra diagnostic benefit, the additional costs and the logistic disadvantages if all patients were tested with CSSS, our results support a policy to treat patients with a negative clinical diagnosis as a patient without dermatophytosis. On the other hand, patients with a positive clinical picture have to be tested using CSSS. This algorithm results in a LR+ of 4.96 and a LR– of 0.56.
Acknowledgments
We thank the 34 GPs for participating in this study, Mr Pierre Stefan of the Laboratory of dermatopathology of the University of Liège for his technical support, Dr Janssens Monique and Mr Gillé Dirk of the Janssen Research Foundation for their technical and scientific support, Dr Van Cutsem J of Janssen Pharmaceutica for his scientific advice, and Mr Blanckaert of ICI Belgium and Mr De Filette of Loctite Belgium for their material support.
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