Diagnostic Tests: Tips from the Dermatologist #1

This is the first in a series of short articles aiming to encourage colleagues in practice to become familiar with performing diagnostic tests. The majority of tests which will be discussed in these articles require no specialist equipment apart from a good microscope. Using these techniques more often will enable you to get a rapid, convenient and inexpensive indication of whether a patient has a certain ectoparasite infestation.

Skin scraping is a useful diagnostic test that dermatologists often do when presented with pruritic patients to rule out the presence of parasites. This test may confirm an infestation but its sensitivity for ruling it out depends on the type of parasite present and aggressiveness of sampling. Patients presenting with pruritus, erythema, scaling, crusting, alopecia, or a papular/pustular eruption should be scraped to rule out trombiculidiasis, cheyletiellosis, sarcoptic or demodectic mange. The presence of scales, crusts on the hocks, elbows and pinnal margins are prime sites for finding Sarcoptes mites. Primary lesions such as papules and pustules associated with pruritus should be scraped for Sarcoptes spp. or Demodex spp. mites. Alopecic areas with comedones and hyperpigmentation should be scraped for Demodex spp mites. Areas with scaling which are very pruritic should be suspicious for Cheyletiella mites.

 Technique

  1. Prior to scraping, hair should be clipped with a number 40 surgical clipper blade.
  2. Gently squeeze the skin between thumb and forefinger if Demodex spp. mites are suspected as this may facilitate their detection by extruding them from hair follicles.
  3. A small amount of liquid paraffin should be applied to the area of the skin to be scraped.
  4. Place some mineral oil on a blunted number 10 or 15 scalpel blade to be used to scrape material from the skin surface. Scrape in the direction of hair growth.
  5. Superficial skin scrapings done on a large area are needed when looking for sarcoptes mites.
  6. Deep skin scrapings with capillary oozing are required when looking for Demodex mites.
  7. The material should be scooped away from the skin and then mounted onto a microscope slide in liquid paraffin.
  8. A cover slip should always be applied to ensure a uniform layer thus the slide is more readily examined
  9. The amount of material on the slide should not be excessive as thick samples are difficult to examine
  10. Lower the condenser of the microscope to improve the contrast
  11. Regulate the light as needed.
  12. Using 4x or 10x objective is enough to see mites.
  13. Scan the slide carefully and thoroughly

Skin scrapes should be taken from three to five different sites. Excessively crusted or excoriated areas should be avoided. Practice and practice and you will be rewarded by finding the mites if they are there!

 

chyletiella mitedemodex sarcoptes