Diagnosing Pneumocystis jirovecii Pneumonia
Study Supports Using Serum β-glucan
By Bill Malone
Pneumocystis jirovecii pneumonia (PCP) caused much of the morbidity and mortality early in the human immunodeficiency virus (HIV) epidemic. However, even as antiretroviral therapy has reduced its prevalence, PCP is still the leading serious opportunistic infection for HIV-positive individuals. A new study examined the use of (1→3)-β-D-glucan (β-glucan) as a diagnostic test for PCP, the subject of this issue of Strategies.
Unlike other fungal infections, PCP cannot be cultured, so diagnosis usually depends on immunofluorescent staining of bronchoalveolar lavage fluid or induced sputum. Currently, nucleic acid amplification tests have been developed that are very sensitive for detecting pneumocystis in respiratory secretions, but they cannot adequately distinguish colonization from disease. Due to the difficulty in obtaining respiratory samples, a serum blood test for PCP has been described as the holy grail for PCP diagnosis. Now a recent study found that plasma β-glucan levels strongly correlated with HIV-related PCP (Clin Infect Dis 2011;53:197–202).
Researchers at Brigham and Women’s Hospital in Boston examined the relationship between plasma levels of β-glucan and HIV-related PCP among 252 participants enrolled in the AIDS Clinical Trials Group A5164, a study of antiretroviral therapy for acute opportunistic infections (OIs), including PCP. Of these, 173 (69%) had received a diagnosis of PCP, making it the largest study to date examining β-glucan for PCP diagnosis. The investigators used a β-glucan assay with a positive threshold of ≥80 pg/mL. Median β-glucan in specimens with PCP was 408 pg/mL versus 37 pg/mL without PCP. The test exhibited a sensitivity of 92% and a specificity of 65%. Positive and negative predictive values were 85% and 80%, respectively. “The strong association of β-glucan with HIV-related PCP could have significant clinical implications, especially in settings where the induced sputum examination for PCP is unavailable or has a low sensitivity,” the authors wrote.
Ironically, as the prevalence of PCP has declined, so has the lab expertise and availability for staining respiratory samples, explained co-author Paul Sax, MD, who is clinical director of the HIV program and the Division of Infectious Diseases at Brigham and Women’s Hospital and associate professor of medicine at Harvard Medical School. “The use of induced sputum to diagnose PCP became common in the late 1980s, so it was something that many labs became real experts on,” he said. “However, over time, as PCP has become less common in HIV patients, that test has become less sensitive, and published reports of its sensitivity vary widely in the range of 40 percent to 60 percent, with only early studies showing sensitivities of 90% or greater.”
Another downside to using respiratory samples is that obtaining them frequent is difficulty for patients, especially those who are very ill, Sax said. “It’s important to remember that the induced sputum test requires the participation of the patient in the process. And if he or she cannot participate in doing the test, then the test doesn’t get a specimen result at all. It’s not that uncommon to have the patient unable to participate for the procedure.” Reasons for non-participation include the patient’s being too ill, unable to cough, or that they find the nebulized saline used to extract the sample too irritating.
Notwithstanding the problems associated with respiratory samples, the β-glucan test has disadvantages of its own, according to the coauthors of an accompanying editorial, Alison Morris, MD, of University of Pittsburgh Medical Center and Henry Masur, MD, of the National Institutes of Health Critical Care Medicine Clinical Center (Clin Infect Dis 2011;53:203-204). “Serum β-glucan provides an interesting window on the biology of fungal pneumonia and the presence of circulating fungal cell wall components. A serologic test for PCP would be a welcomed addition to our ability to diagnose PCP. However, this serum β-glucan test will not yet allow us to lay down the bronchoscope,” they wrote. Morris and Masur emphasized that β-glucan is not specific enough. Most pathogenic fungi have β-glucan in their cell walls and slough off minute quantities into the circulation during the life cycle. As a result, β-glucan is detectable in the serum in cases of several invasive fungal infections.
Sax contended that the specificity of the test in his study (65%) should take into account the cohort of very ill patients. “Actually, one of the strengths of our study is that we applied this test to a group of patients presenting with diverse opportunistic infections, so in a way, the test performance here was in a worst-case scenario. If you had limited the population, solely to those who had respiratory symptoms suggestive of pneumocystis, the predictive value of the test would have been outstanding. In fact we’re doing that analysis in a sub-group of patients with respiratory symptoms and signs, to see, in fact, how good it is,” he said. All the participants in the study had advanced HIV, with a median CD4 lymphocyte count of 26 cells/µL and plasma HIV RNA levels of 5.02 log copies/mL. Many had other OIs, including cryptococcal meningitis (14%) and bacterial pneumonia (9%).
Clinicians also have the ability to easily rule out many other possible fungal infection that could cause a positive β-glucan result, Sax noted. Antigen tests are currently widely available for histoplasmosis and cryptococcosis. In addition, mucosal Candida infection was not, in this study, independently associated with PCP, as many of the patients with oral or esophageal candidiasis also had PCP, Sax said.
With less access to testing of respiratory samples, empiric treatment of PCP has become more common, despite the fact that guidelines issued for managing HIV-related OIs recommend not treating patients presumptively for PCP. “This is a real problem because treating pneumocystis in patients is a fairly big undertaking. It means high doses of co-trimoxazole for 21 days, and when you use those high doses, there is a much higher rate of side effects, so it’s best to know if you’re actually treating PCP that’s there rather than just suspected PCP,” Sax said. At Brigham and Women’s Hospital, the lab now performs the β-glucan test in-house, after Sax and his colleagues found that use of the test reduced both the number of cases treated empirically and the need for bronchoscopy. A U.S. Food and Drug Administration-cleared test kit for β-glucan is available; however, most labs send β-glucan requests to reference labs.
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