1114 LETTERS TO THE EDITOR © 2009 The Authors JEADV 2009, 23, 1092–1115 Journal compilation © 2009 European Academy of Dermatology and Venereology diseases is different, with acrokeratoelastoidosis predominantly manifesting on the palmar surfaces of the hands rather than the dorsa or the sides of the fingers. Although clinically similar at first sight, light microscopic and/or electron microscopic differences in the epidermis as well as the dermis clearly set acrokeratoelas- toidosis apart from FAH, with the former disease typically showing (i) hyperkeratosis located in a cup-shaped epidermal depression overlying a (ii) dermis with characteristic elastorrhexis. 3,8,9 To the best of our knowledge, such histopathologic abnormalities have not yet been described in FAH. Thus, we think that FAH and acrokeratoelastoidosis are separate entities, a notion that is in line with the opinion of other groups. 2,5–7 Finally, we would like to direct the attention of Khamaysi et al. to another issue that arose while considering their contribution. Looking closely at the photographs provided, we note that some nails reveal a V-shaped notching (e.g. digit III of the right hand). Although nail dystrophy is often an unspecific clinical sign, this particular abnormality is consistently observed in Darier’s disease. 10 Furthermore, it seems to us that several dyskeratotic keratinocytes are present. 1 Taking these two aspects into con- sideration, we think that Khamaysi et al. should probably also consider acrokeratosis verruciformis of Hopf (as a sign of Darier’s disease) as a differential diagnosis here. Although the typical ‘church spires’, encountered in acrokeratosis verruciformis of Hopf, are lacking, we would still suggest additional examination (e.g. by further sectioning of available tissue samples). If doubts persist, analysis of the ATP2A2 gene might permit a definitive diagnosis. 10 In conclusion, we believe that there is sufficient evidence to suggest that FAH and acrokeratoelastoidosis are different entities. Since sporadic as well as hereditary variants with autosomal- dominant transmission have been described for both diseases, 2,4 identification of the genetic defect underlying either one of these disorders should eventually resolve the current debate. Acknowledgements van Steensel is supported by grants from the GROW research institute of the University Hospital Maastricht, ZONMW grant 907-00-202, and Barrier Therapeutics NV. Frank is supported in part by a grant to the Network for Ichthyoses and Related Keratinization Disorders (NIRK) from the German Federal Ministry of Education and Research (BMBF). MAM van Steensel,* J Frank Department of Dermatology and Maastricht University Center for Molecular Dermatology (MUCMD) and GROW – School for Oncology and Developmental Biology, University Medical Center Maastricht, The Netherlands *Correspondence: MAM van Steensel. E-mail: mvst@sder.azm.nl References 1 Khamaysi Z, Bergman R, Sprecher E. Nail dystrophy in focal acral hyperkeratosis: a distinctive feature? J Eur Acad Dermatol Venereol 2008; 22: 891–893. 2 Hafner O, Gerstel C. [Focal acral hyperkeratosis]. Hautarzt 1999; 50: 586–589. 3 van Steensel MA, Verstraeten VL, Frank J. Acrokeratoelastoidosis with nail dystrophy: a coincidence or a new entity? Arch Dermatol 2006; 142: 939–941. 4 Rongioletti F, Betti R, Crosti C et al. Marginal papular acrokeratodermas: a unified nosography for focal acral hyperkeratosis, acrokeratoelastoidosis and related disorders. Dermatology 1994; 188: 28–31. 5 Shbaklo Z, Jamaleddine NF, Kibbi AG et al. Acrokeratoelastoidosis. Int J Dermatol 1990; 29: 333–336. 6 Dowd PM, Harman RR, Black MM. Focal acral hyperkeratosis. Br J Dermatol 1983; 109: 97–103. 7 Highet AS, Rook A, Anderson JR. Acrokeratoelastoidosis. Br J Dermatol 1982; 106: 337–344. 8 de Boer EM, van Dijk E. Acrokeratoelastoidosis: a spectrum of diseases. Dermatologica 1985; 171: 8–11. 9 Peterson CM, Lesher JL Jr, Sangueza OP. A unique variant of Darier’s disease. Int J Dermatol 2001; 40: 278–280. 10 Sakuntabhai A, Ruiz-Perez V, Carter S et al. Mutations in ATP2A2, encoding a Ca 2+ pump, cause Darier disease. Nat Genet 1999; 21: 271–277. DOI: 10.1111/j.1468-3083.2009.03218.x XXX Letter to the Editor Letters to the Editor Letters to the Editor LETTERS TO THE EDITOR Autologous serum skin test: an in vivo prozone-like phenomenon? Editor I read with great interest the letter by Husz et al. 1 about serum dilution as a useful tool to avoid false positive results in the autologous serum skin test (ASST). However, I would like to draw attention to one more reason why serum dilution should be used in order to avoid another interesting pitfall when testing with undiluted serum. This may be analogous to the prozone phenomenon observed in vitro in various antibody assays 2,3 or in vivo, for example, in protection against Streptococcus pneumoniae infection where only a narrow dose range of protective antibodies were efficient. 4 In some of our patients with chronic urticaria, ASST with undiluted serum was either negative, or dilution of their sera conferred stronger reactivity (Fig. 1). This has never been seen in healthy subjects or patients with other dermatoses. The dilution dependency we observed with a couple of our patients could be a consequence of high titres of autoantibodies to the FcεRI or IgE – the prozone-like phenomenon. Not only that serum dilution seems to enhance the specificity of the ASST but also it may help avoid false negative results. We continue to perform ASST with serial dilutions and compare the results with clinical course of chronic urticaria. With the forthcoming data by Husz et al., this might improve utility of the valuable easy-to-use test. MD Pavloviç* Dermatology Centre Parmova, Parmova 53, SI-1000 Ljubljana, Slovenia *Correspondence: MD Pavlovi3. E-mail: milos.pavlovic@dcp.si