Tests Diagnose Invasive Aspergillosis With 100% Accuracy
Researchers give ‘thumbs up’ to NASBA/qPCR combo
The fungal infection invasive aspergillosis (IA) can be life threatening, especially in patients whose immune systems have been weakened by chemotherapy or immunosuppressive drugs. Despite the critical need for early detection, IA remains difficult to diagnose.
A study in the September 2014 issue of the Journal of Molecular Diagnostics compared three diagnostic tests and found that the combination of nucleic acid sequence-based amplification (NASBA) and real-time quantitative PCR (qPCR) detects aspergillosis with 100% accuracy. IA is caused by the fungus Aspergillus fumigatus, which is considered by many pathologists to be the world’s most harmful mold. “Traditional diagnostic methods, such as culture and histopathology of infected tissues, often fail to detect Aspergillus,” lead investigator Yun Xia, PhD, said.
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 from 80 patients at high risk of IA. Of these patients, 42.5% had proven or probable IA. The patients came from intensive care, hematology, neurology, nephrology, geriatrics, and other hospital departments.
The tests were evaluated singly and in combination. Individually, NASBA had the highest sensitivity (76.5%), whereas qPCR offered the highest specificity (89.1%). NASBA also was best able to indicate that a patient did not have the infection (negative predictive value). NASBA and qPCR each had a high Youden index (a measure of the effectiveness of a diagnostic marker).
Combining the tests improved the outcomes. The combination of NASBA and qPCR led to 100% specificity and 100% positive predictive value (i.e., the probability that subjects actually had 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,” Xia said. “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 noted that NASBA offers the advantages of rapid amplification (90 minutes) and simple operation with a low instrument cost compared with qPCR and GM-ELISA. They cautioned that although GM-ELISA is routinely used for the diagnosis of aspergillosis, it had low sensitivity (52.9%) with reasonable specificity (80.4%) in the new study, 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 those with advanced acquired immunodeficiency syndrome (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 neurologic problems.