Diagnostic Approaches to Skin Disease

Question 1

Considerations for diagnostic testing

Answer 1

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

Question 2

Diagnostic tests for ectoparasites

Answer 2

Coat brushing
Unstained acetate tape
Skin scrapings (deep and superficial)
Trichograms

Question 3

Diagnostic tests for bacteria

Answer 3

Cytology
Microbial swab for culture

Question 4

Diagnostic tests for fungi

Answer 4

Trichograms (dermatophytes on shaft)
Cytology (yeasts)
Wood’s lamp (Microsporum canis)
McKenzie coat brush (dermatophytes for culture)
Microbial swab for culture (yeasts)

Question 5

Skin surface cytology tests

Answer 5

Used to visualise cells and microbes on skin surface
* Direct impression smear
* Cotton bud impression smear
* Acetate tape strip

Question 6

Type of lesion for direct impression smear

Answer 6

Most/greasy lesions
Ruptured pustule
Skin under crusts
Accessible sites

Question 7

Type of lesion for cotton bud smear

Answer 7

Ear canals

Question 8

Type of lesion for stained acetate tape strip

Answer 8

Dry lesion
Less accessible locations

Question 9

Staining for direct impression smear

Answer 9

All 3 diff quick solutions
A - Fixative
B - Eosin
C - Methylene blue

Dip method

Question 10

Staining for cotton bud smear

Answer 10

All 3 Diff-quick solutions
A - Fixative
B - Eosin
C - Methylene blue

Rack method

Question 11

Staining for acetate tape strip

Answer 11

Only B and C
*Dip method

Or C only
* Pipette

Question 12

Fine needle aspirate cytology

Answer 12

To visualise skin cells below surface

Question 13

Eosinophil granule colour using H&E stain

Answer 13

Pink - Eosinophilic

Question 14

Neutrophil granule colour using H&E stain

Answer 14

Granules don’t stain

Question 15

Basophil granule colour using H&E stain

Answer 15

Dark blue - Basophilic

Question 16

Why do we biopsy the skin?

Answer 16

• To establish a definitive diagnosis that can’t be reached by less invasive testing methods
• To rule out other conditions

Question 17

What tests are carried out on skin biopsies?

Answer 17

Histopathology
* Allows assessment of tissue architecture
* Usually staid with H&E

Tissue Culture
* To culture and identify Bactria/fungi below skin surface

Question 18

When to use sedation/anaesthesia

Answer 18

Can generally carry out biopsies under sedation and local anaesthetic
Use GA for biopsies of feet, lips and noses

Question 19

How many samples should you take?

Answer 19

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

Question 20

Samples for alopecia

Answer 20

○ Across margin of alopecia
○ Area of maximum hair loss
○ Normal haired skin
Wedge shape across margin is useful

Question 21

Samples for ulcerated skin

Answer 21

○ Skin adjacent to ulcer where epidermis is sill intact
Wedge shape across margin is useful

Question 22

Samples for Pustules, vesicles or bullae

Answer 22

Remove whole lesion without disruption
Wedge biopsy is best

Question 23

How to prepare sample site

Answer 23

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

Question 24

How to prepare sample site when using LA

Answer 24

○ 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

Question 25

Types of biopsy

Answer 25

Punch biopsy
Wedge/ellipse
* Excisional
* Incisional

Question 26

Punch biopsies

Answer 26

○ 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

Question 27

Wedge/ellipse biopsy

Answer 27

Tissue excised with a scalpel

Question 28

Excisional biopsy

Answer 28

Used for solitary nodules
Vesicles
Cut all the way around lesion

Question 29

Incisional biopsy

Answer 29

Transition from normal to lesions skin
Biopsy of cutaneous masses
Used if pathology suspend in deep dermis/subcutis

Question 30

How to handle sample appropriately

Answer 30

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

Question 31

Considerations for cutaneous masses

Answer 31

• 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

Question 32

Considerations for bacterial and fungal culture

Answer 32

• 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