Invasive pulmonary fungal disease has an atypical clinical presentation that is easily masked by the underlying disease, and confirming the diagnosis usually requires invasive tissue specimens, a procedure that is often difficult to perform due to the limitations of the patient’s condition. Therefore, there is a high rate of underdiagnosis. At present, most hospitals in China are carrying out non-invasive laboratory techniques such as G test and GM test as one of the diagnostic criteria for fungal infections to improve the positive rate of fungal infections. But how significant are the G test and GM test in the diagnosis of fungal infections? What is the difference between them?
I. What are G test and GM test?
1.G test: also known as 1,3-beta-D glucan test, which detects 1,3-beta-D glucan, a cell wall component of fungi. 1,3-beta-D glucan can specifically activate the G factor in the lysate of horseshoe crab deformed cells, causing the lysate to coagulate, so it is called G test.
2, GM test: the test is galactomannan (glactomannan, GM), galactomannan is a polysaccharide widely present in Aspergillus and Penicillium cell wall, the fungal cell wall surface mycelium growth, galactomannan from the weak mycelium tip release, is the earliest release of antigen, can be detected by enzyme-linked immunosorbent assay.
Second, the significance of both in the diagnosis of fungal infections
According to the diagnostic guidelines for invasive pulmonary fungal disease, the diagnostic factors for fungal infections include host factors, clinical features, microbiological examination, and histopathology. Definitive diagnosis of disease must rely on invasive tests and operations such as histopathology, and the culture process takes time, thus invariably increasing the rate of underdiagnosis. New serological diagnostic methods, including the G test, GM test, and for fungal-specific DNA, PCR techniques, together with clinical signs, microbiological cultures, and especially CT scans, provide the opportunity to start preemptive treatment, monitor the course of disease, and evaluate treatment response more informative information. Among them, 2 consecutive positive G tests and GM tests are meaningful test results.
After phagocytosis of fungi in human body, 1,3-beta-D glucan is continuously released by phagocytes, which increases the level in blood and body fluids. The G-test for 1,3-beta-D glucan is a timely indicator of fungal infection.
G test is suitable for the early diagnosis of all deep fungal infections except for Cryptococcus and Splinterella (Trichophyton). Although more pathogenic fungi including Aspergillus and Candida can be measured, and preliminary clinical studies show good sensitivity and specificity with low false positive rate, it can only indicate the presence or absence of fungal invasive infection, and cannot determine what kind of fungal infection, which is the drawback of this method.
Also false positives can occur in the following cases.
(1) Hemodialysis using fibrin membranes where the specimen or patient is exposed to gauze or other material containing dextran;
(2) Intravenous infusion of immunoglobulins, albumin, coagulation factors or blood products;
(3) Streptococcal bacteremia;
(4) contamination during handling of specimens by the operator. In addition, the use of polysaccharide anticancer drugs, mucosal damage caused by radiotherapy resulting in dextran in food or colonized Candida entering the blood through the gastrointestinal tract may also cause false positives.
GM test is mainly aimed at the early diagnosis of invasive Aspergillus infection. Aspergillus infection site is mainly concentrated in the lungs, thus causing invasive Aspergillus in the lungs. The key to diagnose whether Aspergillus is colonizing or invasive growth in the lungs is whether it synthesizes GM. If the sputum or alveolar lavage fluid specimen is cultured with Aspergillus and the GM test result is positive, the diagnosis of invasive Aspergillus infection is made.
The GM test is often positive 5-8 d before the onset of clinical symptoms, and can be performed on serum, cerebrospinal fluid, alveolar or bronchial lavage fluid, thus often allowing earlier diagnosis. Therefore, the GM test is one of the microbiological evidence for the diagnosis of invasive Aspergillus infection, and the GM value can be used as one of the reference indicators for the effectiveness of treatment.
The GM test is not effective for other fungal tests, and its sensitivity and specificity are affected by many factors.
False positives can occur in the following situations.
(1) Use of semi-synthetic penicillin, especially piperacillin/tazobactam;
(2) Neonates and children;
(3) Hemodialysis;
(4) autoimmune hepatitis, etc;
(5) Consumption of high-protein foods such as milk and contaminated rice that may contain GM, etc.
False negatives may occur in the following cases.
(1) Trichoderma GM (including mannan) released into the blood circulation does not persist but will be cleared quickly;
(2) Previous use of antifungal drugs;
(3) The disease is not severe;
(4) Patients with non-granulocyte deficiency.
Third, clinical application note
(1) G test and GM test detect different substances, metabolic laws differ, and the influencing factors also differ greatly, and they cannot replace each other. The combined application of the two can improve the diagnostic ability of invasive fungal diseases.
(2) Both GM test and G test have false-positive results, and the false-positive rate can be reduced by multiple testing. Therefore, cases with positive G test and GM test still need to be combined with clinical manifestations to determine the presence of fungal infection.
(3) China’s invasive fungal disease guidelines take 2 consecutive positive GM tests as the criteria for microbial infection. Early application of empirical treatment in the clinic will result in lower serum GM concentrations and false negatives, and the criterion of 2 consecutive positive serum GM tests is not easy to achieve, which will bring confusion to clinicians.
(4) The most common factor affecting GM test results is the use of β-lactam antibacterial drugs, especially piperacillin/tazobactam, so such drugs should be avoided for patients when testing for Aspergillus galactomannan.