A total of 4,970 encounters in 4,375 patients had ICD-9 codes compatible with pneumonia. Of those, 3,287 had admission chest radiographs compatible with pneumonia. Of that group a total of 873 patients were excluded for chronic dialysis (58), prior pneumonia within a year (96), prior hospitalization within 90 days (435), or admission DNR/DNI status (333). This yielded a cohort of 2,413 episodes of radiographically confirmed CAP in 2,354 patients (see ). Of this cohort 1,540 (64%) were admitted to the hospital; 1,935 (80%) were evaluated in the ED, with 873 discharged to home from the ED. Of the hospitalized patients, 378 (25%) were admitted to the ICU during the hospital stay; 94 (25%) of these were initially admitted to the hospital ward with later transfer for worsening clinical status. The characteristics of the patient population are displayed in , including mean values for prediction scores, which differed significantly between ward and ICU patients.
Overall 30-day mortality was 3.7% (N=89). Thirty-day mortality was 16.1% (N=61) for ICU patients, 5.6% (N=86) for admitted patients, and 0.3% (N=3) for patients discharged from the ED. The predicted SAPS-2 mortality for the entire population was 12.9%. Of patients admitted to the ward, 855 (74%) received guidelines-compliant antibiotic therapy. Of patients admitted to the ICU, 264 (70%) received guidelines-compliant antibiotic therapy. The most common types of non-compliant therapy for both groups were fluoroquinolone monotherapy (in the ICU) or failure to include coverage for atypical organisms and Legionella in addition to a beta-lactam. The rate of microbiological diagnosis was 17% for ICU patients, 9% for ward patients, and 6% for all-comers. The predominant organisms were S. pneumoniae and S. aureus.
Most pre-admission predictors had few missing values. For hemodynamic predictors, <0.1% were missing. For BUN, 13.8% were missing (4.3% of admitted patients). Less common biochemical results were often missing, including albumin (49% missing among admitted patients) and arterial pH (74% missing among admitted patients). Formal P/F ratios before admission were only available for 13% of patients (37% of those admitted to the ICU). Admission S/F ratios were available for 87% of patients; the remainder lacked simultaneous recorded FiO2 and SpO2.
Of the 378 patients admitted to the ICU, 298 (79%) received an intensive therapy. Of those receiving any intensive therapy regardless of location of care (N=454), 171 (38%) received mechanical ventilation, 46 (10%) non-invasive positive pressure ventilation, 363 (80%) received FiO2≥0.6, 115 (25%) vasopressor therapy, 28 (6%) inotropic therapy, 17 (4%) emergent renal replacement, and 84 (19%) high-volume fluid resuscitation. The 30-day mortality rate for patients admitted to the ICU was 16.1%, while mortality was 13.9% for patients receiving any critical therapy, 27.5% for those receiving mechanical ventilation or vasopressors, and 37% for patients receiving both mechanical ventilation and vasopressors. Rates of receipt of intensive therapy and associated mortality are displayed in .
Therapies received by triage group and mortality
The three reference definitions of SCAP—ICU admission, receipt of intensive therapy, or receipt of intensive therapy with ICU admission—had similar performance in predicting thirty-day mortality. The AUC for the various definitions of SCAP were 0.77-0.78 (95% CI: 0.72-0.83), without significant difference among definitions. Incorporating SAPS-2 predicted mortality into the mortality prediction improved the AUC of the three definitions to 0.89-0.90 (95% CI: 0.85-0.93), again without significant difference among SCAP definitions.
For ICU-admitted patients, log-normalized mean ICU LOS was substantially longer (81 vs. 20 hours) among those who received a critical therapy. Hospital LOS was significantly longer for ICU-admitted patients than for patients admitted only to the hospital ward (6.9 vs. 2.9 days, p<0.01). The log-normalized mean hospital LOS for ICU admitted patients who received intensive therapy was 8.5 days, which was significantly longer (p<0.01) than the 3.9 days for ICU admitted patients who did not receive intensive therapy (excluding patients who died in the ICU). Patients who received an intensive therapy regardless of ICU admission had a hospital LOS of 6.5 days. Linear regression of the log-transformed hospital and ICU LOS for admitted patients had poor fit regardless of SCAP definition employed, including when SAPS-2 predicted mortality was incorporated into the regression model (R2 ~ 0.2 for all models).
Using the reference SCAP definition (receipt of intensive therapy in the ICU) in the subset of patients evaluated in the ED, the IDSA/ATS 2007 criteria performed better (p<0.01) than CURB-65, SMART-COP, and CURXO-80, with an AUC of 0.88 (95% CI 0.85-0.90). Significance remained even after a Bonferroni correction for multiple comparisons. displays the performance of the various predictive models. Neither incorporation of SAPS-2 predicted mortality nor restricting the analysis to admitted patients affected the relative performance of the IDSA/ATS 2007 predictors to a significant degree. Sensitivity analysis using the other two possible definitions of SCAP (ICU admission or receipt of intensive therapy) demonstrated slightly lower AUC, but IDSA/ATS 2007 continued to perform better than the other models. Exclusion of patients who met major criteria at admission did not significantly change results. Consistent with its small AUC, specific CURB-65 cutoffs also performed poorly. Of patients with CURB-65≥ 2, 26% (N=207) were admitted to the ICU and received an intensive therapy, while 54% of patients with CURB-65≥ 4 (N=28) were admitted to the ICU and received an intensive therapy.
Performance of predictive models for SCAP
The probability of SCAP increased in a generally linear fashion with higher numbers of IDSA/ATS 2007 minor criteria, though with a clear stepup at 4 minor criteria, as displayed in Figure 2. The IDSA/ATS 2007 proposed cutoff of 3 minor criteria yielded a PPV for SCAP of 54% and a NPV of 94%. Using 4 minor criteria as a diagnostic cutoff yielded a PPV of 81% and a NPV of 92%. This 27 percentage point increase in PPV is larger than for any other threshold value of minor predictors. The positive likelihood ratio, a Bayesian measure independent of disease prevalence, reached an impressive 34.9 with four criteria. Positive and negative predictive values, sensitivity and specificity, and positive and negative likelihood ratios are displayed in . Exclusion of patients meeting major criteria before admission had only a minor effect on these estimates. Patients (N=41) meeting major criteria (pre-admission mechanical ventilation and/or vasopressor therapy) met on average 3.9 minor criteria, though with a range from 0 to 7 (median 4, inter-quartile range 3-5). Fifteen percent of patients meeting major criteria met two or fewer minor criteria.
Test characteristics for IDSA/ATS 2007 minor criteria prediction of SCAP
The unweighted dichotomized minor criteria (AUC 0.88, 95% CI 0.85-0.90) were inferior by a small but statistically significant margin to the weighted dichotomized minor criteria for IDSA/ATS 2007 (AUC 0.90, 95% CI 0.88 - 0.92), p<0.01 for the comparison. When weighted continuous minor criteria were used, including cubic splines for heart rate, blood pressure (mean) and respiratory rate, the AUC was 0.92 (95% CI 0.90 - 0.94). The results of logistic regression are displayed in . Confusion was the most predictive of SCAP, though tachycardia, hypotension, low P/F ratio, and hypothermia were also predictive. Advanced age was negatively associated with SCAP, likely reflecting lower ICU referral rates for elderly patients independent of admission DNR/DNI documentation. A sensitivity analysis using S/F ratios only (excluding P/F ratios), did not significantly affect the validity of the IDSA/ATS 2007 minor criteria (data not shown).
Weighted, continuous predictors of SCAP among patients evaluated in ED