Critical evaluation of 4-week incubation for fungal cultures: is the fourth week useful?

To determine the benefit of a 4-week incubation for mycology cultures, we evaluated all positive cultures during the fourth week of incubation in a 1-year period. Of 3,855 positive mycology cultures (yeast, 82%; molds, 18%), 62 (1.6%) were positive during the fourth week (yeast, 42%; molds, 58%). Only 15 of the 62 cultures (24%) were considered clinically relevant (2 isolates from invasive fungal infection and 13 isolates from cutaneous mycosis). With the exception of those from skin samples, isolates recovered during the fourth week are rarely important for patient care.