Invasive mycoses are a common type of infection in the clinic and it is important to learn to diagnose them. The clinical manifestations of invasive pneumomal disease are atypical and easily masked by underlying diseases, and the diagnosis usually requires invasive tissue specimens, while invasive procedures are often difficult to implement due to the patient's condition, so there is a high rate of missed diagnoses.
At present, most hospitals in China are carrying out non-invasive laboratory techniques, such as G test and GM test, which have become one of the diagnostic criteria for fungal infections to improve the positive rate of fungal infections.
But how significant is the G test and the GM test in diagnosing fungal infections? What is the difference between the two? Let's take a look at this article one by one.
What is the G test, the GM test?
1, G test: also known as 1,3-β-D dextran test, the detection of the fungus's cell wall components 1,3-β-D dextran, the body's phagocytes engulf the fungus, can continue to release the substance, so that the blood and body fluids in the content increased. 1,3-β-D dextran can specifically activate the G factor in the deformed cell lysate (LAL) of the horseshoe crab, causing the lysate to coagulate, so it is called the G test.
2, GM test: the detection is galactomannan (Glactomannan, GM), galactomannan is a polysaccharide widely present in the cell wall of Aspergillus and Penicillium, when the hyphae on the surface of the cell wall grow, galactomannan is released from the top of the weak hyphae, is the earliest release of antigens, can be detected by enzyme-linked immunosorbent assay.
Significance of both in the diagnosis of fungal infections
According to the Guidelines for the Diagnosis of Invasive Pneumonomycosis, diagnostic factors for fungal infection include host factors, clinical features, microbiological examination, and histopathology. The confirmed disease must rely on invasive examinations and operations such as histopathology, and the culture process takes a certain amount of time, which invisibly increases the rate of missed diagnoses.
New serological diagnostic methods, including G tests, GM tests, and polymerase chain reaction (PCR) techniques for fungal-specific DNA, together with clinical signs, microbial cultures, and especially CT scans, provide more valuable reference data for initiating preemptive treatment, monitoring the course of the disease, and evaluating treatment responses. Among them, two consecutive positive G tests and GM tests are meaningful test results.
After the body's phagocytes engulf the fungus, they can continuously release 1,3-β-D dextran, which increases the content in the blood and body fluids. The G-test to detect the content of 1,3-β-D dextran can reflect the fungal infection in a timely manner.
G test
The G test is suitable for the early diagnosis of all deep fungal infections except Cryptococcus and Zygomycetes (Mucormycetes), although it can detect more pathogenic fungi, including Aspergillus and Candida, and preliminary clinical studies have shown good sensitivity and specificity, and the false-positive rate is low, but it can only indicate the presence or absence of fungal invasive infection, and cannot determine what kind of fungal infection, which is a defect of this method.
False positives can also occur in the following cases:
(1) hemodialysis using cellulose membranes, specimens or exposure of patients to gauze or other materials containing dextran;
(2) Intravenous infusion of immunoglobulins, albumins, coagulation factors or blood products;
(3) streptococcalemia;
(4) There is contamination when the operator handles the specimen.
In addition, the use of polysaccharide anti-cancer drugs, radiation and chemotherapy caused by mucosal damage leading to food dextran or colonized Candida through the gastrointestinal tract into the bloodstream may also cause false positives.
GM test
The GM test is primarily aimed at the early diagnosis of invasive Aspergillus infection. Aspergillus infection sites are mainly concentrated in the lungs, causing invasive aspergillus in the lungs, and the key to diagnosing whether Aspergillus colonizes or grows invasively in the lungs depends on whether it synthesizes GM. Invasive aspergillus infection is diagnosed if sputum or alveolar lavage samples are cultured with Aspergillus and the GM test is positive.
GM tests often yield positive results 5 to 8 days before the onset of clinical symptoms and can test serum, cerebrospinal fluid, alveoli, or bronchial lavage fluid, often prediagnosing. Therefore, the GM test is one of the microbiological examination evidence for the diagnosis of invasive Aspergillus infection, and the detection of GM value can also be used as one of the reference indicators of treatment effect.
GM testing is ineffective for other fungal tests, and sensitivity and specificity are affected by a number of factors, false positives can occur in the following cases:
(1) the use of semi-synthetic penicillin, especially piperacillin / tazobactam;
(2) newborns and children;
(3) Hemodialysis;
(4) Autoimmune hepatitis, etc.;
(5) Eat high-protein foods such as milk that may contain GM and contaminated rice.
False negatives can also occur in the following cases:
(1) Aspergillus GM (including mannan) released into the blood circulation does not persist but will be cleared quickly;
(2) Previous use of antifungal drugs;
(3) The condition is not serious;
(4) Patients with nongranulocytosis.
Clinical application note
(1) The substances detected by the G test and the GM test are different, there are differences in metabolic laws, and the influencing factors are also quite different, the two cannot be replaced by each other, and the combined application of the two can improve the diagnostic ability of invasive mycosis.
(2) There are false positive results in both the GM test and the G test, and the false positive rate can be reduced by multiple tests. Therefore, cases of positive G test and GM test still need to be combined with clinical manifestations to determine the presence of fungal infection.
(3) The mainland's "Diagnostic Guidelines for Invasive Pulmonary Mycosis" takes 2 consecutive GM positives as the standard for microbial infection, and the early application of clinical empiric treatment will reduce the serum GM concentration, false negative, and the standard of 2 consecutive serotonergic GM is not easy to meet, which will bring confusion to clinicians.
(4) The most common factor affecting the results of GM test is the use of β-lactam antibacterial drugs, especially piperacillin/tazobactam, so patients should be avoided when detecting aspergillus galacerin mannan.
Source: Respiratory Channel of the Medical Profession
Editor: Ren Mileage Reviewer: Xiao Ran
