IgM examination for respiratory diseases

Respiratory disease IgM examination is a rapid auxiliary screening method used clinically to determine the acute phase infection of respiratory pathogens. The core value is to indicate the possibility of recent infection. However, a single result cannot be used as the gold standard for diagnosis and must be comprehensively judged in conjunction with clinical symptoms, imaging and other test results.

Last week I was attending the fever clinic and met a little girl who was in her second year of high school. She had coughed for three days and had a low fever of 37.8 degrees. As soon as her parents entered the door, they handed over a community-checked IgM positive report for Mycoplasma pneumoniae and asked, "Doctor, do we need an infusion? Do we need to take three courses of treatment?" I checked that her lungs were fine and her blood test was normal, so I prescribed some mild cough medicine to let her go home to rest. I didn't even prescribe antibiotics. Her parents looked worried, so they came back for a follow-up checkup three days later. The little girl's fever had basically subsided and her cough was mostly relieved.

In fact, many people’s misunderstandings about this test stem from not understanding what IgM is. You can think of it as the "advanced scouts" sent by the immune system. Whenever foreign pathogens such as viruses and mycoplasmas invade, the first antibody produced by the immune system is IgM. Generally, it can be detected in the blood 3-5 days after infection. When the "main force" of IgG comes up in about 2 weeks, IgM will slowly disappear. So in theory, if IgM is positive, it basically means that you have been exposed to the corresponding pathogen in the past 1-2 weeks, and you are most likely in the acute stage of infection.

In our clinical circles, there have always been two different tendencies regarding the positioning of IgM examinations. Many outpatient doctors particularly like to prescribe this test. After all, it can produce results quickly. It can be tested with fingertip blood. If it is quick, the report can be obtained in 40 minutes. Compared with nucleic acid, which has to wait for three or four hours and throat swabs, which are more troublesome, it is suitable for outpatient clinics during the peak season of respiratory infections in autumn and winter. It is very practical for primary screening - when the mycoplasma epidemic occurred at the end of last year, we relied on IgM rapid screening to divert highly suspected patients to specialized clinics, which greatly reduced the probability of cross-infection and allowed patients in need to use symptomatic drugs as early as possible. Speaking of which, during peer exchanges last year, some doctors from community hospitals joked that they would rather prescribe more IgM than nucleic acid. After all, many patients, especially children, are very resistant to throat swabs. Instead, they are willing to cooperate with a fingertip blood draw. Even if the accuracy is not good, it is much better than patients directly refusing to take the test.

However, many experts in laboratory and respiratory departments have been reminding not to rely too much on IgM results. Its false positive and false negative rates are actually not low. I met an aunt with Sjögren's syndrome a while ago. She had a fever and coughed up yellow sputum. The test for mycoplasma IgM was strongly positive. After taking azithromycin for five days, she didn't feel better at all. Finally, she did a lower respiratory tract nucleic acid test and found out that it was an Acinetobacter baumannii infection. It had nothing to do with mycoplasma. It was an antibody cross-reaction caused by her autoimmune disease, and the test showed a false positive. Some people have just been infected for 1-2 days, and IgM has not had time to be generated, so they are found to be negative. As a result, they run around without taking it seriously, and come back two days later when the fever reaches over 39 degrees, which in turn delays the time. It is precisely because of this that many tertiary hospitals have now replaced IgM rapid screening with the more accurate enzyme-linked immunoassay test, or directly recommended that suspected patients check nucleic acids simultaneously to reduce the possibility of misjudgment.

After all, ordinary patients don’t need to be overly nervous when they get an IgM report. Don’t feel like the sky is falling and have to take a bunch of medicine when you see a positive result, and don’t let it go completely when you see a negative result. If you don’t have any symptoms at all, but your physical examination turns out to be positive by the way, you should go to work and go to school, just drink plenty of warm water and take a good rest, and there is no need for special treatment at all. If you have typical respiratory symptoms such as fever, cough, and sore throat, leave it to the doctor to determine whether to combine chest X-ray, blood routine, or nucleic acid for further confirmation. Do not take antibiotics at home.

I usually ask about the symptoms before I go out for an IgM test. If you have been coughing for more than a week, I will usually directly recommend that you check the IgG together to see if the antibody titer has doubled. The results are much more accurate than a single IgM test. To put it bluntly, this checklist is just like the estimated delivery time when you order takeout. It can only be used as a reference. The specific time when you can get the meal depends on whether the rider is blocked on the road and whether the merchant is delivering the meal, right? After all, no matter what kind of examination, the ultimate goal is to help people get better quickly. The most important thing is not to be tied up by a report sheet.