Respiratory disease IgM test

By detecting pathogen-specific IgM antibodies in the serum, it is determined whether the subject is in the active stage of infection. It has a high reference value for the early identification of common respiratory pathogens such as influenza virus, Mycoplasma pneumoniae, new coronavirus, adenovirus, and respiratory syncytial virus. However, it cannot be used alone as the gold standard for diagnosis. The results need to be combined with clinical symptoms, imaging examinations, or nucleic acid/antigen testing for comprehensive judgment.

Respiratory tract infections are highly prevalent during this period. When many people go to hospital fever clinics or pediatricians, they have encountered situations where doctors prescribe this test. A tube of venous blood is drawn, and the results can be obtained in more than an hour. It is much more convenient than waiting for nucleic acid tests for a long time, and it is more specific than just looking at blood routines.

Friends who know something about immunity may know that IgM is the first "vanguard" mobilized by the human immune system in response to foreign pathogens. It can generally be detected in the blood 3-7 days after infection, which is much earlier than IgG antibodies that only appear 1-2 weeks after infection. Moreover, once the infection enters the recovery period, the IgM titer will quickly drop and turn negative, so its positive result can basically point to a "recent infection" rather than an old infection a few months ago.

However, on the clinical frontline, there are actually different opinions on the value of this examination. Doctors in the laboratory department are more willing to recommend it. After all, the cost is low and the throughput is high. Especially for pathogens like Mycoplasma pneumoniae that take more than ten days to be isolated and cultured traditionally, IgM testing is almost the fastest specific reference result available in outpatient and emergency departments. However, clinicians in the respiratory department and pediatrics department tend to be more cautious in interpreting the results. When I went to the outpatient clinic with the director of pediatrics last month, I met a parent who came with a weak positive report for mycoplasma IgM. The baby's fever subsided after a day, with only a slight cough, and there were no rales in the lungs. The director directly told him to go home and drink more water for observation, without even prescribing Archie. After asking, I found out that the baby had just contracted influenza A half a month ago, and it was likely that the previous infection had triggered an immune cross-reaction, resulting in a false positive.

The problems of false positives and false negatives are indeed shortcomings that cannot be avoided in IgM testing. Patients with autoimmune diseases such as rheumatoid arthritis and lupus erythematosus, or people in the acute stage of other infections, may have positive results that have no reference meaning. ； On the other hand, if the infection time is too short, and blood is drawn just 1-2 days after the fever occurs, and the antibodies have not yet risen to the threshold for detection, false negatives and missed diagnoses will also occur. Many experts now advocate that antigen or nucleic acid should be used first for outpatient and emergency screening, which is more accurate. However, at the peak of medical treatment, nucleic acid has to wait for several hours, and the sensitivity of antigens varies. IgM has become the most cost-effective compromise option.

In fact, we ordinary people don’t need to panic when we get the IgM report sheet. Don’t rush to infuse and take antibiotics when you see a positive result. If your symptoms are very mild, just a sore throat, low fever, and no chest tightness, severe cough, or difficulty breathing, you can observe yourself at home for 2 days. Many respiratory virus infections are self-limiting and can be overcome by your own immunity. If you have a high fever that persists for more than 3 days, or you cough so much that you can't sleep at night, or you have severe breathlessness after activities, you can go to the doctor with the report and take a chest X-ray or CT scan, and if necessary, do a nucleic acid review, and you can basically get a clear diagnosis.

To put it bluntly, the respiratory IgM test is more like a "warning light". If it lights up, it does not mean that there must be a serious problem. If it does not light up, it does not completely rule out infection. There is no need to regard its results as the only criterion for judgment. It is safest to follow the doctor's advice and respond based on your own symptoms. Oh, by the way, you don’t need to be fasting to do this test. You can take it anytime you go to the hospital. You don’t have to go there hungry.~