Cultures should always be obtained from all biopsies and aspirates. The utility of cultures for the immediate diagnosis of IFI is limited because it takes time for fungi to grow in vitro, often too long to wait for a critically ill HSCT patient. Blood cultures can be negative, even in patients with fungemia and IFI. Cultures from BAL fluid have been found to yield positive results in only about 50% of cases of invasive aspergillosis,46 and in even fewer patients with pulmonary zygomycosis.47
On the other hand, when cultures are positive, results may simply represent colonization or contamination, not true infection. However, a positive culture from respiratory tract secretions has a high positive predictive value in immunocompromised HSCT patients10, 46 and infection should be presumed and treated until proven otherwise.47,48 The presence of highly suggestive clinical manifestations, radiographic findings, and multiple positive cultures increases the positive predictive value of positive respiratory cultures.46
Surveillance cultures are now rarely used; they are too costly and have variable utility. Although their negative predictive value is high, their positive predictive value is low.42 The utility of such cultures is largely dependent on the degree of risk in the patient population. As an example, in a recent study involving 92 neutropenic patients from whom weekly nasal swabs were collected, none of those with negative swabs subsequently became infected, but only 6.4% of those with a positive Aspergillus culture developed infection.49 Despite the low positive predictive value, however, a 6.5-fold increase in the risk of invasive pulmonary aspergillosis accompanied the presence of a positive nasal swab,49 a statistical finding that supports the need for preemptive antifungal therapy in immunocompromised patients with positive respiratory cultures.
Course Number: V035D
This CME Expires on July 1, 2005; no tests will be accepted after this date.
This course is accredited by
The University of Pittsburgh School of Medicine, Center for Continuing Education
and The International Immunocompromised Host Society