Combined Test Detects Deadly Fungus
By combining two methods, scientists have developed a more sensitive and specific test for invasive aspergillosis.
AsianScientist (Aug. 20, 2014) - A study in The Journal of Molecular Diagnostics shows that the combination of nucleic acid sequence-based amplification (NASBA) and real-time quantitative PCR (qPCR) detects the fungal infection invasive aspergillosis (IA) with 100 percent accuracy.
IA is caused by the fungus Aspergillus fumigatus, which is considered by many pathologists to be the world's most harmful mold. IA can be life threatening, especially in patients whose immune systems are weakened by chemotherapy or immunosuppressive drugs. Despite the critical need for early detection, IA remains difficult to diagnose.
"Traditional diagnostic methods, such as culture and histopathology of infected tissues, often fail to detect Aspergillus," comments lead investigator Dr. Xia Yun, of the First Affiliated Hospital of Chongqing Medical University.
In this retrospective study, scientists evaluated the diagnostic performance of two nucleic acid amplification assays (qPCR and NASBA) and one antigen detection method (galactomannan enzyme-linked immunosorbent assay [GM-ELISA]) using blood samples collected from 80 patients at high risk of IA. Of the 80 patients, 42.5 percent had proven or probable IA.
The tests were evaluated singly and in combination. Individually, NASBA had the highest sensitivity (76.47 percent) whereas qPCR offered the highest specificity (89.13 percent). NASBA also was the test that best indicated that a patient did not have the infection (negative predictive value).
Combining the tests improved the outcomes. The combination of NASBA and qPCR led to 100 percent specificity and 100 percent positive predictive value (the probability that subjects truly have the infection).
"Because each test has advantages and disadvantages, a combination of different tests may be able to provide better diagnostic value than is provided by a single test," says Dr. Xia.
"The combination of NASBA and qPCR should be useful in excluding IA in suspect cases, thus reducing both suffering and expense for immunocompromised patients. On the other hand, the combination of NASBA and qPCR could be more suitable for screening patients suspected of infection, because this assay had the highest sensitivity."
The authors note that NASBA offers the advantages of rapid amplification (90 minutes) and simple operation with low instrument cost compared with qPCR and GM-ELISA. They caution that although GM-ELISA is widely and routinely used for aspergillosis diagnosis, this study indicates that it had low sensitivity (52.94 percent) with reasonable specificity (80.43 percent), making it "inferior to both NASBA and qPCR."
The A. fumigatus mold is ubiquitous in the environment and is found on decaying plant matter. For healthy individuals exposure to the fungus can be inconsequential, but it can cause significant morbidity and mortality for those with compromised immune systems, including patients who have undergone organ transplants or have advanced AIDS.
Even patients with more modest immune impairments, such as diabetes, poor nutrition, steroid use, or lung disease, can become severely infected. Symptoms may include fever, cough, difficulty breathing, chest pain, seizures, and focal neurological problems.
Source: Elsevier Health Sciences; Photo: Yale Rosen/Flickr/CC.
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