Total Number of Cases that do not Result in Death
The total number of cases per year was obtained from the literature on skin and skin structure infections. Using data from the Healthcare Cost and Utilization Project National Inpatient Sample (HCUP NIS) for the 5-year period from 2000 to 2004, Edelsberg, et al. (2009) estimated the total number of hospital admissions for SSTIs to be 869,777 in 2004. The authors also provided estimates broken down by type of skin infection, allowing us to exclude chronic ulcers and infections, gangrene, necrotizing fasciitis, decubitus ulcer infections, diabetic foot infections, and certain healthcare-associated infections (as specified in the FDA guidance for ABSSSIs) and arrive at an estimated 678,956 hospital admissions for ABSSSIs, or approximately 231.9 per 100,000 population. Applied to the 2011 U.S. population, this rate is equivalent to an estimated 726,321 inpatient ABSSSI cases for the year 2011.14
We calculated the total number of patients hospitalized with ABSSSIs that do not die in a given year by subtracting those who die in that year from the total number of ABSSSI patients. This number can then be subtracted from the estimated total number of hospital admissions for ABSSSI in 2011, 726,321 (explained above), to get 724,397 surviving ABSSSI hospital patients.
QALYs Lost per Case
To calculate lost QALYs for patients who have an ABSSSI but recover, we first searched the Tufts database and found a QALY weight of 0.642 for cellulitis, abscess, and wound infection, three major types of ABSSSI (though this QALY weight was for hospital patients infected with MRSA and therefore might represent cases on the more serious end of the severity spectrum) (Lee, et al., 2010). As with ABOM, we then adjusted the QALY weight by period of illness.
The average length of inpatient stay for patients hospitalized for an SSTI is 6.1 days (Menzin, et al., 2010). However, many skin infection patients are treated in both inpatient and outpatient settings, and length of hospital stay does not capture additional days spent sick or recovering outside the hospital. The mean number of days of episode duration is 24.4 days, which includes time spent in both inpatient and outpatient treatment settings (Marton, et al., 2008). Using 24.4 days as the illness period, we calculated the lost QALYs per patient to be 0.02393 as:
Total QALYs Lost due to Morbidity
Given that lost QALYs per ABSSSI case is 0.02393 and the total number of ABSSSI cases that do not result in death in the US is around 724,397, we computed the total annual QALYs lost due to ABSSSI morbidity to be 17,336 in the US.
To calculate VSLY-based illness costs (for patients who do not die), we first calculated an average VSLY weighted by ABSSSI incidence by age group (available from Edelsberg, et al., 2009). This weighted average VSLY is roughly $365,500, which is then multiplied by the average lost QALYs per patient (0.02393) to arrive at $8,749 per patient. The total morbidity cost due to ABSSSI is then $6.3 billion (= $8,749 × 724,397) per annum.
14 The total number of ABSSSI cases—including patients who are treated in the outpatient setting only—is, however, far greater. Hersh, et al. (2008) examined visits by patients with SSTIs to physician offices, hospital outpatient departments, and emergency departments using NAMCS and NHAMCS and found that the overall rate of visits for SSTIs was 48.1 visits per 1000 population in 2005, totaling 14.2 million visits. As patients with skin infections are likely to visit these healthcare settings multiple times over the course of their SSTI episode, it is necessary to divide the total number of visits by the average number of visits per episode to arrive at the number of episodes per year. According to Marton, et al. (2008), who analyzed skin and skin structure infections caused by Staphylococcus aureus using managed care claims data for the years 2002-2005, the mean number of physician visits per episode was 6.3. Thus, 14.2 million outpatient visits divided by 6.3 visits per episode equals roughly 2.3 million episodes per year in 2005, or 778.1 per 100,000 population. Thus, the estimates presented herein constitute a lower bound.