The data of patients with MPP who were admitted to Women and Children’s Hospital of Ganzhou from May 2019 to July 2021 were retrospectively collected. The inclusion criteria were as follows: 1) met the diagnostic criteria: with clinical manifestations (presence of fever, cough, tachypnea, and difficulty in breathing), abnormal lung auscultation, radiologic findings (presence of a new infiltrate on chest radiography or consolidation not attributable to some other etiologies), and MP detected to be positive by laboratory tests (Medjo et al., 2014); and 2) age less than 15 years. The exclusion criteria were as follows: 1) children with evidence of coinfection; 2) children with immune deficiency; and 3) children with a history of allergies. Severe MPP (SMPP) was defined as MPP with any one of the following: 1) a poor general condition; 2) fastidium or dehydration; 3) disturbance of consciousness; 4) an increased respiratory rate (infants >70 breaths/min and older children >50 breaths/min); 5) dyspnea; 6) cyanosis; 7) extent of infiltration on chest X-ray ≥2/3 of one lung or multilobe involvement; 8) extrapulmonary complications; 9) pleural effusion; and 10) oxygen saturation in room air ≤92% (Zheng et al., 2021).

On the day of admission, 3 ml of venous blood was collected from patients, and the passive agglutination assay was used to detect MP antibody (MP Antibody Test Kit, Fujitsu Joint Stock Company). It was defined as MP positive when the MP antibody titer was ≥1:160. Nasopharyngeal aspirates were collected on the day of admission, and MP-DNA was detected by PCR (MP Nucleic Acid Test Kit, National Sun Yat-sen University). Simultaneous detection of other pathogens was performed through the indirect immunofluorescence assay of respiratory viruses (adenovirus, respiratory syncytial virus, parainfluenza virus 1–3, and influenza virus A and B) using a D3 Ultra DFA Respiratory Virus Screening & ID Kit (Diagnostic Hybrids, Inc., OH, USA) and blood cultures for bacteria (BD FX200 blood culture system).

By consulting the electronic medical records of all patients, the demographic, clinical, and laboratory data were collected retrospectively. Clinical signs and symptoms of patients, including fever, cough, wheezing, chest pain, and complications, were obtained. All patients underwent chest X-ray or chest CT scan to confirm focal or segmental infiltration, with or without pleural effusion, atelectasis, pneumothorax, pulmonary embolism, and pulmonary necrosis.

Serum IgE levels were obtained using the automatic biochemical immunoassay analyzer produced by Roche. The test kit was also provided by Roche. The reaction was carried out according to the manufacturer’s instructions.

This study was approved by the Ethics and Research Council of Women and Children’s Hospital of Ganzhou (201905-A01) on May 15, 2019. The data from patients were collected anonymously.

The statistical analyses were carried out using SPSS 20.0 software package. Continuous variables were reported as the mean ± SD and were compared using Student’s t-test or the non-parametric Mann–Whitney U-test. The categorical variables were shown as number (%) and were compared using the chi-square test. p-Values <0.05 were considered to be significant. The figures and graphs were generated by GraphPad Prism 5.

From May 2019 to July 2021, 275 patients admitted to our hospital for MPP were enrolled in the study. The demographic and clinical characteristics are shown in Table 1 . Of these, 167 (60.7%) were male and 103 (39.3%) female. The median age of the children was 4.93 years, varying from 2 months to 14 years 10 months; 9.4% of the patients were under 12 months, 24% of the patients were between 1 and 3 years, 32.4% of the patients were between 3 and 6 years, and 34.2% of the patients were older than 6 years. A total of 97 patients had SMPP. Almost all the children had cough (99.6%) and fever (95.3%), 24 (8.7%) had wheezing, and 22 (8.0%) had chest pain. Moreover, 98 (35.6%) patients received glucocorticoid therapy, 70 (25.5%) patients received electronic bronchoscopy alveolar lavage, 31 (11.3%) patients required oxygen therapy, 11 (4%) patients received immunoglobulin therapy, and eight (2.9%) patients needed intensive care unit (ICU) treatment. Further, 95 (34.5%) children experienced pulmonary complications, and 52 (18.9%) children experienced extrapulmonary complications.

The pulmonary and extrapulmonary complications are listed in Table 2 . The IgE levels of patients with different complications varied from 0 to 2,500 IU/ml. Complications occurred in 147 patients, of which pulmonary complications were more common than extrapulmonary complications. Thirty (10.9%) patients had atelectasis, and 55 (20%) had pleural effusion, accounting for 89.5% of pulmonary complications. Extrapulmonary complications were more common in terms of skin rashes and hypokalemia. Further, 21 (7.64%) patients had hypokalemia, 12 (4.4%) patients had skin rashes, six (2.18%) patients had myocarditis, and six (2.18%) patients had hepatitis. Patients with complications of necrotizing pneumonitis, pneumothorax, skin rashes, eye pain, or bronchiolitis obliterans had higher IgE levels. No statistically significant difference was found in IgE levels between pulmonary complications and extrapulmonary complications with p = 0.292, as shown in Figure 1 .

Patients were divided into two groups according to the presence or absence of complications. The demographics, clinical characteristics, laboratory findings, and treatments of children in the complication and non-complication groups are shown in Table 3 . The children in the complication group were much older than those in the non-complication group (p = 0.001). No statistically significant difference in the appearance of fever and cough symptoms was observed between the two groups. Higher incidence of chest pain (p = 0.000) and lower incidence of wheezing (p = 0.017) were found in the complication group than in the non-complication group. Significant differences in fever duration, hospitalization duration, and incidences of SMPP between the two groups (p < 0.001) were also found. The probability of SMPP in patients with complications was higher than that in patients without complications (p = 0.000). As shown in Figure 2 , the IgE level fluctuated greatly from patient to patient. The IgE level in the complication group was higher than that in the non-complication group (p = 0.041). The risk factors for complications by multifactor binary logistic regression analysis were analyzed. After age factors were adjusted, a significant difference in the IgE level was noted between the complication and non-complication groups (p = 0.047), as shown in Table 4 . The threshold of IgE between these two groups was determined using the receiver operating characteristic (ROC) curve. The area under the ROC curve was 0.671. When the cutoff value for the IgE level was set at 245.6 IU/ml, the specificity and sensitivity were 75.51% and 52.27%, respectively ( Figure 3 ). Besides the IgE level, D-dimer, lactate dehydrogenase (LDH), and ferritin levels also differed significantly between the two groups (p < 0.01). However, no significant difference was observed in white blood cell count, neutrophil percentage, hemoglobin, platelets, erythrocyte sedimentation rate, and the levels of immunoglobulin A, immunoglobulin G (IgG), immunoglobulin M, alanine transaminase (ALT), aspartate transaminase (AST), and creatine kinase isoenzymes (CK-MB). Patients in the complication group were more likely to receive oxygen therapy, glucocorticoids, and bronchoscopy as compared with those in the non-complication group (p < 0.01). Meanwhile, eight patients required ICU treatment, and one required mechanical ventilation. Except for one child who died, the rest of the children recovered and were discharged from the hospital.

There were some patients with abnormally increased IgE (cases with IgE above 1,000 IU/ml) in our study. The information of these 14 patients is listed in Table 5 . These patients were all acutely infected, mainly manifesting with fever and cough. There were four children younger than 3 years: two of them had wheezing, and one had complication of bronchiolitis obliterans. Two of the 14 children with elevated IgE had no complication. It was found that the older the patients, the more likely to have complications. Pulmonary complications were more common in patients with high IgE, and prognosis was good after active treatment. But unfortunately, of these 14 patients, one died due to multiple complications.