Introduction

Broad-range PCR assays for fungi are molecular diagnostic methods designed to detect a wide spectrum of fungal pathogens directly from clinical samples such as blood, BAL, sputum, CSF, tissue biopsies, and sterile body fluids.
These assays target highly conserved fungal genomic regions, primarily the Internal Transcribed Spacer (ITS) and 18S/28S rRNA genes, enabling identification of yeasts, molds, dimorphic fungi, and even rare or unculturable species.

Broad-range fungal PCR is especially valuable in critically ill, immunocompromised, and oncology/transplant patients, where a rapid diagnosis can be lifesaving and traditional culture is slow or yields negative results.

Principle

Broad-range PCR relies on universal fungal primers that amplify conserved regions shared among diverse fungi.

1. DNA Extraction

Fungal DNA is extracted directly from various samples:

• BAL, sputum
• Blood, serum/plasma
• CSF
• Tissue biopsies (fresh or FFPE)
• Peritoneal, pleural, or vitreous fluids

2. PCR Amplification

Universal primers (e.g., ITS1/ITS4, 18S/28S rRNA) amplify fungal DNA present in the specimen.

3. Detection/Analysis

Amplified products are detected by:

• Conventional PCR + Gel electrophoresis

• Real-Time PCR
• Nested PCR
• LAMP assays

4. Sequencing for Identification

Sanger sequencing of PCR products provides species-level identification through comparison with databases such as:

• NCBI GenBank
• UNITE
• MycoBank

Clinical Significance

1. Rapid Diagnosis of Fungal Infections

Detects fungi within hours, much faster than culture (which may take days to weeks).

2. Detects Difficult or Unculturable Fungi

Helpful when the culture is negative due to:

• Low fungal load
• Prior antifungal therapy
• Slow-growing / non-viable / fastidious fungi
• Deep-seated or tissue-restricted infections

3. Essential for Immunocompromised Patients

Widely used in:

• Oncology
• Transplant recipients
• ICU patients
• HIV/AIDS
• Neutropenic individuals

4. Broad Coverage of Pathogens

Identifies:

• Candida spp.
• Aspergillus spp.
• Mucorales
• Fusarium
• Cryptococcus
• Histoplasma
• Trichophyton
• Rare environmental fungi

5. Useful in Sterile-Site Infections

Positive results from CSF, cardiac tissue, vitreous fluid, bone, or joint fluid indicate true infection.

6. Helps Guide Early Antifungal Therapy

Early detection supports targeted treatment and reduces mortality.

7. Resolves Mixed or Polymicrobial Infections

Sequencing reveals the presence of multiple fungi in the same specimen.

Keynotes

• ITS region is the gold standard fungal barcode for broad-range PCR detection.
• Direct PCR is highly sensitive, but may detect colonizers; interpret results with clinical context.
• Requires strict contamination control (separate pre- and post-PCR areas).
• Sequencing confirmation is essential for species identification.
• Cannot replace culture for antifungal susceptibility testing.
• Best used with an integrated approach, including:
  • Culture
  • Microscopy (KOH, Calcofluor, GMS/PAS)
  • Antigen tests (GM, BDG)
  • Radiology (CT/MRI)
• Useful for FFPE samples, where culture is not possible.

Further Readings

1. https://www.mdpi.com/2079-6382/13/12/1166
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC8525558/
3. https://bmcmicrobiol.biomedcentral.com/articles/10.1186/s12866-016-0752-1
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC2845394/
5. https://www.medrxiv.org/content/10.1101/2023.08.02.23293145.full
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC3592065/
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC5934447/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC9863793/
9. https://www.pharmacytimes.com/view/fungal-biomarkers-primer-for-pharmacists