Germ tube test

Principle

• Candida albicans (and C. dubliniensis) produce germ tubes, the start of true hyphae, when incubated in serum at 35–37 °C for a short time.
• A germ tube is a short outgrowth with parallel sides and no constriction where it joins the parent cell.

Specimen

• Pure yeast colony from a fresh culture (e.g. 24–48 h on Sabouraud dextrose agar).

Reagents and equipment

• Pooled human serum, rabbit serum or fetal bovine serum (about 0.5 mL per test), small test tubes, pipettes, slides, coverslips, incubator or water bath at 35–37 °C.

Procedure

• Lightly touch a colony with a sterile loop or pipette tip and suspend it in the serum; a heavy inoculum inhibits germ tube formation.
• Incubate at 35–37 °C for 2–3 hours (do not exceed 3 hours).
• Place a drop on a slide with a coverslip and examine with 40× objective.

Results

• Positive: short, slender filaments arising from the yeast cells without constriction at the point of origin (often about 3–4 times the length of the yeast cell).
• Negative: yeast cells only, or pseudohyphae showing a constriction at the base.
• Presumptive identification of C. albicans (C. dubliniensis also positive).

Quality control and pitfalls

• Run with each batch: positive control C. albicans and negative control such as C. tropicalis or Nakaseomyces (Candida) glabrata.
• Incubation beyond 3 hours allows C. tropicalis to form pseudohyphae that can be misread as germ tubes.
• A small percentage of C. albicans strains are germ-tube negative.
• Heavy inoculum or old serum can give false negatives; mixed cultures give misleading results.
• C. auris is germ tube negative and needs MALDI-TOF or molecular identification.

Clinical use

• Rapid, inexpensive presumptive identification of C. albicans from clinical isolates.