ObjectiveThis study aimed to evaluate the predictive value of combining nucleated red blood cells (NRBC) with the pediatric critical illness score (PCIS) in infants with severe pneumonia at 28 days.MethodsThis retrospective study was conducted at the Pediatric Intensive Care Unit (PICU) of the First People's Hospital of Bijie and included infants with severe pneumonia from September 1, 2021, to August 31, 2022. Demographic, clinical, and laboratory data were extracted from electronic medical records, and the differences between the groups were compared. Variables showing significant differences were included in binary multivariable logistic regression analysis to explore independent risk factors for mortality in infants with severe pneumonia. A receiver operating characteristic (ROC) curve was used to evaluate the predictive value of patient outcomes at 28 days.ResultsThe study included 128 patients; at 28 days, 96 survived (survivor group), and 32 had died (nonsurvivor group). Compared with the survivor group, the nonsurvivor group had lower levels of platelets [348 (239–496) versus 431 (324–540) ×109/L; p = 0.023], albumin [35.7 ± 5.2 vs. 37.9 ± 4.4 g/L; p = 0.022], CD3 [36.9 ± 1.7 vs. 47.6 ± 1.8%; p < 0.001], CD4 [20.4 ± 1.2 vs. 23.6 ± 1.9%; p < 0.001], IgG [4.9 ± 1.0 vs. 6.4 ± 0.7 g/L; p < 0.001], IgA [0.5 ± 0.1 vs. 0.8 ± 0.1 g/L; p < 0.001], 25‐hydroxyvitamin D(25(OH)D) [21.1 ± 1.0 vs. 30.7 ± 1.36 ng/mL; p < 0.001], PCIS [72.8 ± 7.8 vs. 87.5 ± 6.2; P<0.001], and a greater NRBC level [0.450 (0.162–0.832) vs. 0.185 (0.100–0.500) ×109/L; p = 0.005]. Binary multivariable logistic regression analysis revealed that the NRBC count (odds ratio (OR) = 2.46, 95% confidence interval [CI] 1.427–4.239; p = 0.001) and PCIS (OR = 0.775, 95% CI: 0.644–0.934; p = 0.008) were independent predictors of 28‐day survival in infants with severe pneumonia. The PCIS had an area under the curve (AUC) of 0.782 (95% CI: 0.680–0.884), NRBC had an AUC of 0.719 (95% CI: 0.622–0.816), and when the NRBC and PCIS were combined, the AUC was 0.929 (95% CI: 0.880–0.978).ConclusionThe NRBC count and PCIS are risk factors for the prognosis of infants with severe pneumonia, and their combination can increase the predictive value of patient outcomes at 28 days.