MP is an important cause of pediatric CAP and is estimated to be responsible for 10–40% of all CAP cases [3]. In China, there are up to 2.7 million pediatric severe CAP every year [6], with 13.0–42.6% of cases being severe MPP [11]. This study assessed the clinical characteristics of pediatric patients with acute MPP to evaluate the risk factors and their predictive values for severe MPP. To date, there is limited evidence on the age, sex, and seasonal characteristics of MPP. A 10-year longitudinal study in North China found that the peak age of MPP was 6–10 years old, accounting for 75.2% of all cases in children [11]. However, in our study, the majority of patients with MPP were infants or preschool-aged children. This finding is consistent with a study based in Thailand [12]. Regional differences in the season of onset have been demonstrated, with increased incidence in the fall and winter in North China, and increased incidence in the summer and fall in South China [11]. Moreover, previous studies have reported an association between MPP and elevated atmospheric temperatures and humidity, with a 17% increase in MPP cases for every 1 °C increase, and a 4% increase in MPP cases for every 1% humidity increase [6, 13]. In the present study, patients lived in Central and South China, and most MPP cases occurred in the spring, summer, and fall, similar to previous studies [6, 13]. Nevertheless, no significant differences in age, sex, and seasonal onset between patients with general MPP and severe MPP were observed in this study. Further large-scale, multicenter studies are needed to validate these findings.
The most common symptoms of MPP were fever and cough. Fever was reported in 73.1% of patient with severe MPP and 57.8% of patients with general MPP. While not statistically significant, fevers tended to be higher in patients with severe MPP. Most children with severe MPP had moderate or high fevers, whereas most children with general MPP had moderate or low fever. The majority of the patients with severe MPP had a fever lasting more than 7 days, a cough lasting more than 2 weeks, and most required hospitalization for more than 7 days. Patients with general MPP had significantly shorter fever durations, cough durations, and hospitalizations compared to those with severe MPP, consistent with previous reports [8, 14, 15]. The Japanese Respiratory Society’s scoring system provides guidance for recognizing MPP, with white blood cell (WBC) count < 10 × 109/L being one of six standard parameters [3]. In our study, patients with MPP had WBC counts below this threshold. Patients with severe MPP had significantly increased levels of neutrophils, CRP and PCT, as well as reduced levels of lymphocytes and platelets compared to patients with general MPP. These findings are consistent Yan et al. [8, 16, 17]. Previous studies have suggested that thrombocytopenia is an extrapulmonary manifestation of MPP [3, 18]. Therefore, acute-phase thrombocytopenia may have predictive value for severe MPP. In general, the presence of pyrogenic products of inflammation and excessive inflammatory response can cause lung damage or even lead to necrotizing pneumonia [19–22].
In recent years, several studies have investigated the risk factors for MPP, but few have assessed their predictive value. Our ROC curve analyses showed that increased PCT levels (> 0.67 ng/mL), prolonged cough duration (> 13.5 days), and heat peak above 38.4 °C may be early predictors for severe MPP in children. The analyses revealed that PCT offers slightly greater predictive value than cough duration and heat peak. Therefore, a detailed history of cough duration and heat peak may be required when assessing the risk for severe MPP [17]. In addition, PCT had greater specificity compared to the other two risk factors. PCT is a 116 amino acid protein encoded by the CALC-1 gene, which typically exists in low concentrations in parafollicular cells of the thyroid gland in healthy individuals. In the presence of an infection, CALC-1 gene expression is induced in non-neuroendocrine cells, resulting in increased PCT levels [23]. As a clinical biomarker of bacterial infections, PCT levels are measured to assess the severity and prognosis of CAP, acute respiratory distress syndrome, and sepsis, and guide antibiotic and glucocorticoid treatment [23–27]. While several studies have investigated the relationship between PCT and Streptococcus pneumoniae, Haemophilus influenzae, and other common bacterial pathogens in patients with CAP, few clinical studies have assessed this relationship in patients with MPP [28]. PCT levels increase within 2–4 h after infection and peak within 24–48 h [29]. The PCT assessment has several advantages including its availability, ease of use, low cost, universality, and reliability. Moreover, in this study, the threshold value for PCT was 0.67 ng/mL, which is much lower than the threshold for severe bacterial infections and sepsis (≥ 2 ng/mL). These findings are consistent with those of previous study that suggested that PCT levels may be used to predict severe MPP in patients with acute MPP [16]. In addition, PCT levels at admission were associated with longer fever duration and hospitalization, indicating that PCT may be a risk factor for prolonged fever in MPP [30].