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Introduction to Dermatological Tests

Recent studies show that skin and ear diseases comprise 25% of all veterinary consultations. They are often complex and ongoing conditions that are a challenge to manage. Very few can be diagnosed on history and appearance alone. The modern approach to dermatology emphasises using the history and clinical signs to construct a logical differential diagnosis list. The diagnosis is then achieved by utilising appropriate tests to eliminate and/or confirm conditions in the differential diagnosis. On the other hand it is all too easy to become over-reliant on tests; it is most important that the clinical pathology is made to fit the history and clinical signs, not vice versa.

There is a very wide range of tests that can be used to investigate skin problems, but selecting, performing and interpreting the most appropriate tests in each case requires some experience. Many textbooks and journals, however, concentrate on individual skin conditions and assume that the reader is experienced enough to undertake the relevant diagnostic procedures. In practice, some of the most common reasons for poor management of skin conditions involve the inappropriate use of diagnostic tests, suboptimal execution of test procedures, inadequate sample choice and misinterpretation of results. The aim of this book, therefore, is to provide an illustrated, step-by-step guide to help you select, perform and interpret clinical tests and procedures for a range of dermatological presentations.

What Equipment Will You Need?

A wide range of equipment is necessary for thorough examination of the skin, which may seem daunting. The vast majority, however, are inexpensive, non-specialist items that are common to virtually all veterinary practices and do not need special skills to operate. The few items that are expensive and/or need specific training to use are all optional; they are undoubtedly useful, especially to dermatology specialists, but are not necessary to successfully practise veterinary dermatology.

Essential Equipment

• Good lighting is essential for proper examination of the skin, lesions and collected material. Good fluorescent room lighting is a minimal requirement and a high-intensity spotlight is necessary for any serious examination.
• Flea comb for coat combings (Figure 1.1).
• Hand lens or magnifying glass for close examination of the skin, coat and collected material; the large illuminated lenses sold for reading are most useful (Figure 1.2).
• A good-quality binocular microscope for examining hair plucks, skin scrapes and cytology (Figure 1.3).
• Glass slides for mounting material; frosted slides are easier to label (Figure 1.1 and Figure 1.4).
• Cover-slips are essential for any microscopic examination (Figure 1.4).
• Immersion oil for using the ×100 oil immersion microscope lens; different types of oil with different viscosities are available. Type A is the least viscous and is often preferred as it is the least messy and the cheapest. Type NVH is the most viscous. For in-house use, type B would also be suitable also but it is messier than type A (Figure 1.4).
• Lens tissue or cloth and cleaning fluid or alcohol for cleaning microscope lenses without damage.
• Otoscope for examining the ears (Figure 1.5).
• Wood's lamp for screening for fluorescent dermatophytes (Figure 1.2).
• Electric clippers for removing hair, allowing access to the skin.
• Curved scissors for more precise and less traumatic hair removal.
• Fine-tipped curved artery (mosquito) forceps for hair plucks (Figure 1.1).
• Liquid paraffin for skin scrapes (Figure 1.1).
• 20% potassium hydroxide as a clearing agent when looking for parasites or dermatophytes (optional).
• No. 10 and No. 15 scalpel blades for skin scrapes and biopsies (Figure 1.1).
• Cotton buds for collecting material from the ears (Figure 1.6).
• Adhesive tape (Sellotape®, Scotch Tape® etc.) for skin surface parasites and cytology (Figure 1.1).
• Sterile bacteriology swabs with and without transport media; fine-tipped ENT swabs are useful for taking samples from narrow sites or when using otoscopes etc. (Figure 1.6).
• Sterile universal (30 mL) and bijoux (5 mL) containers for storing and transporting tissue samples (Figure 1.7).
• Diff-Quik® type stain for routine cytology (Figure 1.4).
• Toothbrushes to collect material for dermatophyte culture (Figure 1.8).
• Syringes and needles of various sizes for taking blood samples and aspirates (Figure 1.9).
• Blood collection tubes: plain, EDTA, heparin and gel-clotting tubes.
• Indelible marker pen to mark biopsy and skin test sites.
• 4-, 6- and 8-mm skin biopsy punches (Figure 1.7).
• Basic surgical kit and suture material for performing skin biopsies and closing skin wounds.
• 10% neutral buffered formalin for fixing biopsy specimens (Figure 1.7).

Figure 1.1 Some equipment needed for taking samples for skin parasitology: a flea comb, clear sticky tape, No. 10 scapel blades, cotton buds, liquid paraffin, artery forceps for taking hair pluckings and microscopic slides.

Figure 1.2 A magnifying lens with illumination. In this case the lamp doubles up as a Wood's lamp for dermatophyte detection.

Figure 1.3 A good-quality microscope is essential to be able to correctly identify parasites, cells and microorganisms in the samples examined. The image shows a trinocular microscope with the option to attach a camera to document the findings.

Figure 1.4 Essential microscope equipment: glass slides with a frosted edge for easy labelling, glass cover-slips to improve the optic performance under the microscope, immersion oil and a modified Romanowsky-type rapid staining solution kit.

Figure 1.5 Equipment used for otoscopy: a handheld otoscope, various sizes of cones to be attached to the otoscope (all packaged individually after autoclaving to sterilise the cones after each use) and a cone cleaner to remove otic debris after use prior to sterilising.

Figure 1.6 Equipment for obtaining samples from the ear canal: cotton buds, bacteriology swabs with transport medium, liquid paraffin and glass slides.

Figure 1.7 Some equipment for obtaining and transporting tissue samples (skin biopsy specimens): single-use biopsy punches in varying sizes and sterile containers with formalin saline for histopathology or empty for tissue culture.

Figure 1.8 Materials to obtain samples for dermatophyte culture/perform in house cultures: clear sticky tape (for direct microscopy), liquid paraffin (for direct microscopy), sterile toothbrushes, artery forceps for hair plucks, glass slides for direct microscopy and a combined dermatophyte test medium (DTM) and Sabouraud agar plate for culture.

Figure 1.9 Materials for fine-needle aspirates: different gauge sterile single-use needles, 5-ml syringes and glass slides.

Optional Equipment

• Dermatophyte test medium for in-house dermatophyte culture.
• Lactophenol cotton blue is useful for staining dermatophyte colonies for identification if in-house cultures are performed.
• Special laboratory stains for cytology: Grams, Leishman, Giemsa, Ziehl-Nielsen etc.
• Intradermal allergen test kit for more experienced clinicians with an interest in dermatology to perform allergy testing in atopic dermatitis (Figure 1.10).
• Video-otoscope (Figure 1.11).

Figure 1.10 Allergens for intradermal allergy testing. The relevant allergens will vary regionally.

Figure 1.11 A video-otoscope is very helpful for performing deep ear flushes.

Use and Abuse of the Practice Microscope

No piece of equipment is so vital or subject to so much abuse as a microscope. Robust and inexpensive models with binocular lenses and integral light sources are easily mastered and give good results, but need looking after and must be used correctly for the best results.

Which Microscope Should I Buy?

Monocular microscopes or, even worse, those with mirrors for external lights, belong in a museum. The ideal microscope should have binocular eyepieces (with one capable of independent focusing), an integral light source, a focusing condenser, a mechanical stage, coarse and fine focus, and four lenses - ×4, ×10, ×40 and ×100 oil immersion (eyepieces are usually ×10 giving a final magnification of ×40 to ×1000). Dry (non-oil) ×60 lenses with a final magnification of ×600 are sometimes used instead of the oil immersion lens. Wide-field, stand-off eyepieces with rubber cups are best, as they can also be used when wearing glasses by folding the rubber cups down. The light source should provide white light (e.g. by using an LED) or have a daylight filter to convert the yellow-orange tungsten light to daylight (i.e blue-white). More expensive microscopes have a filter mount above the light source so that a variety of other filters can be used, although...