Diagnostic Approaches to Skin Disease Flashcards
Considerations for diagnostic testing
Prioritise which tests will help you narrow down differentials
Take an adequate number of good quality samples
Examine efficiently
Be aware of limitations
* False positives
* False negatives
Diagnostic tests for ectoparasites
Coat brushing
Unstained acetate tape
Skin scrapings (deep and superficial)
Trichograms
Diagnostic tests for bacteria
Cytology
Microbial swab for culture
Diagnostic tests for fungi
Trichograms (dermatophytes on shaft)
Cytology (yeasts)
Wood’s lamp (Microsporum canis)
McKenzie coat brush (dermatophytes for culture)
Microbial swab for culture (yeasts)
Skin surface cytology tests
Used to visualise cells and microbes on skin surface
* Direct impression smear
* Cotton bud impression smear
* Acetate tape strip
Type of lesion for direct impression smear
Most/greasy lesions
Ruptured pustule
Skin under crusts
Accessible sites
Type of lesion for cotton bud smear
Ear canals
Type of lesion for stained acetate tape strip
Dry lesion
Less accessible locations
Staining for direct impression smear
All 3 diff quick solutions
A - Fixative
B - Eosin
C - Methylene blue
Dip method
Staining for cotton bud smear
All 3 Diff-quick solutions
A - Fixative
B - Eosin
C - Methylene blue
Rack method
Staining for acetate tape strip
Only B and C
*Dip method
Or C only
* Pipette
Fine needle aspirate cytology
To visualise skin cells below surface
Eosinophil granule colour using H&E stain
Pink - Eosinophilic
Neutrophil granule colour using H&E stain
Granules don’t stain
Basophil granule colour using H&E stain
Dark blue - Basophilic
Why do we biopsy the skin?
- To establish a definitive diagnosis that can’t be reached by less invasive testing methods
- To rule out other conditions
What tests are carried out on skin biopsies?
Histopathology
* Allows assessment of tissue architecture
* Usually staid with H&E
Tissue Culture
* To culture and identify Bactria/fungi below skin surface
When to use sedation/anaesthesia
Can generally carry out biopsies under sedation and local anaesthetic
Use GA for biopsies of feet, lips and noses
How many samples should you take?
Sample a representative range of lesions
Take a minimum of 3 unless solitary lesion
Sample fully developed primary lesions
* Avoid traumatised skin and necrotic crust
Samples for alopecia
○ Across margin of alopecia
○ Area of maximum hair loss
○ Normal haired skin
Wedge shape across margin is useful
Samples for ulcerated skin
○ Skin adjacent to ulcer where epidermis is sill intact
Wedge shape across margin is useful
Samples for Pustules, vesicles or bullae
Remove whole lesion without disruption
Wedge biopsy is best
How to prepare sample site
Don’t disturb skin surface
○ Clip hair with scissors
○ Do not disturb crust
○ Do not prep or scrub
Unless excisional biopsy of sub cut nodules
How to prepare sample site when using LA
○ Draw circle around lesion in indelible marker
○ Infiltrate LA into subcutis around periphery of circle
○ Do not exceed maximum volume of LA for patients weight
○ Check efficacy of analgesia by pricking with needle
Types of biopsy
Punch biopsy
Wedge/ellipse
* Excisional
* Incisional
Punch biopsies
○ Quick and convenient
○ Use 6-8mm biopsy punches routinely
3/4mm for delicate structures
○ Don’t destroy/shear sample
Rotate in one direction only
Hold perpendicular to skin surface
Do not reuse blunt punches
Wedge/ellipse biopsy
Tissue excised with a scalpel
Excisional biopsy
Used for solitary nodules
Vesicles
Cut all the way around lesion
Incisional biopsy
Transition from normal to lesions skin
Biopsy of cutaneous masses
Used if pathology suspend in deep dermis/subcutis
How to handle sample appropriately
Never handle dermis or epidermis
* Only handle by subcutis
For histopathology
○ Blot blood gently from underside of sample
○ Place promptly into 10% formalin
Minimum 10x volume of tissue sample
○ If thin sample
Place on stiff card or end of wooden tongue depressor to prevent curling
○ Give pathologist brief history and suspected differential diagnoses
Considerations for cutaneous masses
- May not be homogenous
○ Especially if large
○ Incisional or excisional likely to be more representative than punch - If suspect invasive neoplasm
○ Take incisional biopsy fist before attempting removal
Helps ensure adequate margins taken when removal performed
Considerations for bacterial and fungal culture
- Used for deep and superficial pyoderma, deep fungal infections
○ Less affected by environmental contamination than surface sample - Withdraw antibiotics for 5-7 days, topical antimicrobials 3+ days prior to sampling for bacterial culture
