What are the diagnostic criteria and methods for diagnosing thrush?

How to Diagnose Thrush (Oropharyngeal Candidiasis)

Thrush is primarily a clinical diagnosis based on characteristic white patches on oral mucosa that can be scraped off, but microbiological confirmation with swabs cultured on fungal selective media is essential for species identification and to guide treatment in recurrent or complicated cases. 1

Clinical Diagnosis

Key Diagnostic Features

• Visual appearance: Look for white adherent patches or plaques on the oral mucosa (pseudomembranous candidiasis), which can be readily scraped with a tongue depressor to obtain samples 1
• Distribution patterns: Examine the buccal mucosa, tongue, palate, and oropharynx for characteristic lesions 1
• Associated symptoms: Document presence of local pain, discomfort, loss of taste, and aversion to food 2
• Variant presentations: Recognize that thrush can manifest as hyperplastic candidiasis, atrophic (denture) candidiasis, linear gingival erythema, median rhomboid glossitis, or angular cheilitis 3

Important Clinical Caveat

Thrush isolated to the larynx can occur without oral manifestations, presenting with hoarseness rather than typical oral symptoms, and is often misdiagnosed for an average of 6 months. 4 This variant lacks dysphagia or odynophagia and may lead to unnecessary surgical intervention if not recognized 4.

Microbiological Confirmation

Specimen Collection and Processing

• Swab technique: Take swabs directly from the lesion by scraping the white patches 1
• Culture requirements: Inoculate swabs on fungal selective media to avoid overgrowth by colonizing bacteria 1
• Microscopy: Observe for pseudohyphae, though note that not all Candida species form filaments during infection (e.g., C. glabrata shows only yeast cells) 1
• Species identification: This is mandatory for all isolates to guide appropriate therapy 1

When to Perform Microbiological Testing

Species identification and antifungal susceptibility testing are recommended in recurrent/complicated cases and in patients who have been exposed to azoles previously. 1

Biopsy Considerations

• Not routinely required: A biopsy is not mandatory for typical presentations 1
• When indicated: Biopsy can discriminate between infection and colonization, or when diagnosis is uncertain 1
• Processing: If obtained, tissue must be placed in fixative rapidly and examined with special stains such as silver stains and PAS 1
• Immunohistochemistry: Can confirm infection when yeasts are seen in tissue but cultures are negative, using genus-specific antibodies 1

Risk Factor Assessment

Identify Predisposing Conditions

• Immunosuppression: HIV infection (particularly CD4+ counts <200 cells/μL), immunosuppressive medications, neutropenia 1, 4
• Medication history: Inhaled corticosteroids (can be the sole causative factor), systemic steroids, broad-spectrum antibiotics 4
• Metabolic conditions: Diabetes mellitus 4
• Local factors: Denture use, xerostomia 3

In HIV-infected patients with CD4+ lymphocyte counts <200 cells/μL or those with counts >200 cells/μL with unexplained fever, weight loss, or thrush, suspect opportunistic infections including Pneumocystis pneumonia and tuberculosis. 1

Diagnostic Algorithm

1. Clinical examination: Identify characteristic white patches that scrape off
2. Obtain swab: Scrape lesion and inoculate on fungal selective media
3. Assess risk factors: Document immunosuppression, medication use, diabetes
4. Species identification: Mandatory for all culture-positive cases
5. Susceptibility testing: Perform in recurrent cases or prior azole exposure
6. Consider biopsy: Only if diagnosis uncertain or to exclude other pathology

Common Diagnostic Pitfalls

• Assuming all white oral lesions are thrush: Leukoplakia and erythroplakia have malignant potential and require biopsy for definitive diagnosis 5
• Missing laryngeal thrush: Consider in patients with isolated hoarseness and risk factors, especially inhaled steroid use 4
• Treating without confirmation: In recurrent or treatment-resistant cases, always obtain cultures to identify non-albicans species and guide therapy 1
• Ignoring underlying immunodeficiency: Persistent or recurrent thrush warrants investigation for HIV, diabetes, or other immunocompromising conditions 1, 4

Special Populations

HIV-Infected Patients

• Use CD4+ counts to construct differential diagnoses for patients with thrush 1
• Clinical diagnosis alone is insufficient in immunocompromised patients with atypical presentations—laboratory confirmation is needed 6

Recurrent or Complicated Cases

• Mandatory testing: Species identification and susceptibility testing must be performed 1
• Consider non-albicans species: C. glabrata and other species may be azole-resistant 1