We used 10 years of weighted data totaling 370.33 million hospitalizations for the analyses. The V09 codes used to identify antibiotic resistance along with their cumulative 10-year frequency are shown in the . Infections resistant to penicillins, including MRSA, accounted for 94.27% of all observed antibiotic-resistant infections. Among hospitalizations with antibiotic-resistant infections, 99.63% only listed one V code, which would correspond to one class of antibiotics.
Cumulative 10-year frequency of specific antibiotic-resistant infection in hospitals: U.S., 1997–2006
The infection diagnosis classification indicating an infection-related hospitalization accounted for 94.96% of total hospitalizations with an antibiotic-resistant infection. The percentage of total hospitalizations with an antibiotic-resistant diagnosis demonstrated a positive exponential relationship (λ=0.161, r-square=0.983) increasing from 0.12% in 1997 to 0.46% in 2006 (p<0.001). The number of hospitalizations with antibiotic-resistant infections increased 327%, from 41,581 (95% confidence interval [CI] 33,994, 49,167) in 1997 to 177,457 (95% CI 161,302, 193,611) in 2006 (data not shown).
The percentage of infection-related hospitalizations with antibiotic resistance also exhibited an exponential relationship (λ=0.151, r-square=0.983), as the yearly tangent line steadily increased and the hospitalization percentage rose from 0.66% in 1997 to 2.40% in 2006 (p=0.001), for a combined 264% increase during the 10-year span. Similarly, the number of infection-related hospitalizations with antibiotic resistance increased from 37,005 (95% CI 30,063, 43,946) in 1997 to 169,985 (95% CI 154,355, 185,614) in 2006.
As shown in , when stratified by age, the hospitalization percentages with antibiotic-resistant infections represent a rise in hospitalizations, with resistant infections becoming more common in younger patients, most notably those <18 years of age. The percentage of infection-related hospitalizations with antibiotic-resistant infections for subjects ≥65 years of age increased by 48.8%, with a weak linear relationship (m=0.046, r-square=0.642) from 0.731% to 1.088% (p=0.01). On the other hand, the percentage of antibiotic-resistant infections for subjects 18–64 years of age increased substantially, with an even more marked increase in patients <18 years of age. In the middle-age group, the proportion increased exponentially (λ=0.207, r-square=0.982) by 435%, from 0.68% in 1997 to 3.64% in 2006 (p=0.001). The pediatric age group indicated an even stronger exponential increase (λ=0.370, r-square=0.972), rising from 0.21% to 5.67% during the 10-year duration. The number of infection-related hospitalizations for those ≥65 years of age with antibiotic resistance increased from 23,185 (95% CI 14,134, 32,235) in 1997 to 41,942 (95% CI 24,795, 59,088) in 2006. Likewise, the number of infection-related hospitalizations for the combined younger age groups increased from 13,820 (95% CI 9,710, 17,929) in 1997 to 128,043 (95% CI 114,792, 141,293) in 2006.
Percentage of infection-related hospitalizations with antibiotic-resistant microorganisms, grouped by age: U.S., 1997–2006
The mean age of individuals with infection-related hospitalizations that had antibiotic-susceptible infections increased slightly from 60.23 years (SE=0.37) in 1997 to 62.07 years (SE=0.14) in 2006. In contrast, the mean age of individuals with infection-related hospitalizations that had antibiotic-resistant infections decreased substantially, from 65.67 years (SE=2.01) in 1997 to 44.20 years (SE=1.47) in 2006 (p=0.001).
The incidence and impact of cellulitis among infection-related hospitalizations was substantial. In 1997, 10.90% of infection-related hospitalizations had a diagnosis of cellulitis with a rise to 15.77% of infection-related hospitalizations by 2006. Among hospitalizations with cellulitis diagnoses, the proportion with resistant organisms rose from 1.19% in 1997 to 8.95% in 2006. This can be contrasted with hospitalizations for infection diagnoses other than cellulitis, of which 0.60% had resistant organisms in 1997 but had risen only to 1.17% by 2006.
The median length of stay for infection-related hospitalizations without indication of antibiotic resistance remained relatively flat and linear (m=–0.05, r-square=0.93) with a bit of a decline, changing from 4.77 days (SE=0.50) in 1997 to 4.29 days (SE=0.45) in 2006. In regard to the length of stay for infection-related hospitalizations with indication of antibiotic resistance, the length of stay rose at first and then began a decline in 1999–2000, totaling an overall 40% decline from 1997 to 2006, with median stays of 6.62 (SE=0.85) and 3.97 (SE=0.24) days, respectively.
plots the median length of hospital stay against the presence of health insurance, suggesting that length of stay in the hospital is closely related to the presence of health insurance. As the proportion of patients with infection-related hospitalizations with antibiotic resistance who did not have insurance increased, the length of stay for those hospitalizations exhibited a corresponding decrease (r=0.91, p<0.01).
Median length of stay in hospital and percent insured, with and without antibiotic resistance: U.S., 1997–2006
Patients who had infections with antibiotic-resistant organisms who did not have health insurance had shorter median lengths of stay (4.15 days, SE=0.25) during the 10-year period than did their counterparts with insurance (5.49 days, SE=0.18). The log-transformed mean length of stay was significantly shorter for those without insurance (p=0.001). A similar effect was found for these patients who had infections with susceptible organisms. Those without insurance had shorter median lengths of stay (3.25 days, SE=0.31) than patients with insurance (4.56 days, SE=0.30). Patients without insurance had a significantly shorter transformed mean length of stay (p=0.001). In terms of cellulitis hospitalizations, these patients without insurance had shorter median lengths of stay (3.01 days, SE=0.08) during the 10-year period than did patients with insurance (4.09 days, SE=0.31). Patients without insurance had a significantly shorter transformed mean length of stay (p=0.001)
Among hospitalizations with an antibiotic-resistant infection, we examined where the patients were discharged. The percentages for 1997–1998 for routine discharge, death, other, and discharge to long- or short-term-care institution were 47.01%, 6.79%, 11.88%, and 34.32%, respectively (). By 2005–2006, the proportion of discharges that were routine had risen to 70.62% (p<0.01), and deaths had dropped to 1.61% (p<0.01). Routine discharges increased by 50.2% during the study period, while death discharges decreased by 76.3% and discharges to care institutions declined by 49.2% during the 10-year observation period.
Discharge status of hospitalizations with antibiotic-resistant infections: U.S., 1997–2006