Dermatology Workup

---
name: dermatology-workup
description: Diagnostic ladder (cytology→scraping→DTM→biopsy), pruritus scoring, common differentials by species (atopy, flea allergy, food allergy, Demodex, Sarcoptes, Malassezia), treatment tiers.
---

# Dermatology Workup

## Overview

Evidence-based diagnostic progression from non-invasive to invasive techniques. Includes pruritus quantification (VAS, CPAT), differential diagnosis by species, Malassezia management, and multimodal treatment strategies.

## When to Use

- User presents pruritic/alopecic dermatologic cases (dogs, cats, exotics)
- User needs diagnostic escalation guidance or differential diagnosis
- Keywords: pruritus, dermatitis, Demodex, Sarcoptes, atopy, FAD, food allergy, Malassezia, cytology, DTM, biopsy, seborrhea

## Diagnostic Ladder

**1. Cytology** (First-line, <24 hours)
- Acetate tape or swab, Diff-Quik staining; yields cocci, yeast, bacteria
- **Malassezia**: Peanut-shaped organisms; overgrowth suggests predisposition (atopy, otitis)
- Limitation: Cannot detect Demodex/Sarcoptes

**2. Skin Scraping** (Second-line)
- Deep scrape (until bleeding) for Demodex; superficial for Sarcoptes/Cheyletiella
- Sensitivity: Sarcoptes 50-70%, Demodex 70-80%; repeat if high suspicion
- Diagnostic threshold: >5 Demodex mites/low-power field

**3. Dermatophyte Test Medium (DTM)**
- Culture 2-3 weeks; Microsporum canis (most common) produces red/pink color
- Sensitivity 70-80%; negative result does not exclude ringworm
- Confirmatory: Sabouraud agar or MALDI-TOF for definitive species ID

**4. Skin Biopsy** (Definitive)
- 6-8 mm punch from active lesion margin; special stains (PAS, GMS) for fungi
- Indicated: Refractory cases, suspected autoimmune, chronic scaling, neoplasia
- Sample multiple sites increases diagnostic yield

## Pruritus Scoring

**Visual Analog Scale (VAS)**: 0-10 owner-reported (0 = none, 10 = severe); trending more important than absolute
**Canine Pruritus Assessment Tool (CPAT)**: Scratching bouts, biting frequency, sleep disruption, lesion severity
**Feline**: Grooming frequency, alopecia, self-trauma patterns

## Common Differentials by Species

### Dogs
1. **Atopic Dermatitis**: 10-15% prevalence; onset 1-3 years; face/ears/neck/paws; **diagnosis**: clinical (parasites/food/infection ruled out)
2. **Flea Allergy Dermatitis (FAD)**: Single flea triggers reaction; caudal dorsum, tail, rear limbs; **MANDATORY** flea prevention trial
3. **Food Allergy**: 8-12 week elimination diet (novel protein); year-round (non-seasonal); concurrent GI signs common
4. **Demodex canis**: Localized vs. generalized; indicator of immunosuppression; requires treatment + underlying disease investigation
5. **Sarcoptes scabiei**: 50-70% diagnostic sensitivity; severe pruritus disproportionate to lesions; trial therapy justified

### Cats
1. **Atopy**: Often presents as over-grooming/miliary dermatitis; self-trauma common; exclusion diagnosis
2. **FAD**: Miliary dermatitis pathognomonic; single flea sufficient; strict flea control mandatory
3. **Food Allergy**: 8-12 week elimination (novel protein); presents with pruritus or GI signs
4. **Demodex cati**: Rare; generalized mange uncommon; associated with FIV/FeLV/immunosuppression
5. **Sarcoptes**: Rare in cats; presumptive diagnosis often necessary

## Malassezia Considerations

**Epidemiology**: Lipophilic yeast; overgrowth pathogenic in atopy, seborrhea, otitis, maceration
**Predisposed breeds**: Basset hounds, Cocker spaniels, Poodles, Shih Tzus
**Diagnosis**: Cytology (Diff-Quik); culture not standard (normal flora)
**Treatment**: Address underlying cause (critical); topical miconazole/terbinafine 2-4 weeks; systemic fluconazole 5-10 mg/kg daily (dogs)

## Treatment Tiers

**Tier 1**: Environmental control (flea prevention all household animals), novel protein trial, bathing, humidity reduction
**Tier 2**: Topical azoles (Malassezia), antibacterial (chlorhexidine), antihistamines (cetirizine, hydroxyzine 10-30% efficacy)
**Tier 3**: Systemic corticosteroids (0.5-1 mg/kg BID initial, taper essential); cephalexin 22-33 mg/kg BID (pyoderma, 2-4 weeks)
**Tier 4**: Cyclosporine 5-10 mg/kg BID (dogs/cats, 4-6 week onset); oclacitinib (JAK inhibitor, 24-48 hour onset); lokivetmab (IL-31 antibody, canine-specific)
**Tier 5**: Specialist referral for IDST, allergen immunotherapy, advanced dermatopathology

## Key Species Differences

- **Dogs**: Corticosteroids effective but taper essential; JAK inhibitors preferred for rapid pruritus control; cyclosporine for long-term
- **Cats**: Diabetes risk with corticosteroids; cyclosporine/JAK inhibitors preferred; elimination diet often diagnostic for food allergy

## Limitations

- Negative skin scrape does not exclude parasitic disease (50-80% sensitivity); repeat or trial therapy justified
- Negative DTM does not exclude dermatophytosis; clinical judgment essential
- Atopy/FAD/food allergy remain clinical diagnoses (exclusion-based); no gold-standard test exists
- Underlying immunosuppression must be identified before treating secondary Demodex
- Referral to board-certified dermatologist recommended for refractory, chronic, or suspected autoimmune cases