Total Number of Cases that do not Result in Death
Estimates of the number of cases of CIAIs each year in the U.S. were not readily available in the literature. Therefore, we estimated incidence using the rate of 2.3 cases of secondary intra-abdominal infections (sIAI) per 10,000 person-years reported in a Netherlands-based study (Sturkenboom, et al., 2005). Using a database of pharmacy dispensing records from community pharmacies linked to hospitalization records, the authors identified potential cases of sIAI on the basis of a primary discharge diagnosis with one of the following International Classification of Diseases, ninth revision (ICD-9-CM) codes: 475, 540–543, 562, 567, 569, 574–577, 614.5, 997.4, 998.2, E8782, and E8783. The authors then excluded “all potential cases that did not receive an intra-abdominal surgical intervention to establish the diagnosis sIAI or antibacterial drug treatment during their hospital admission.” Finally, a clinician and an epidemiologist reviewed the hospital discharge letters associated with the remaining cases to verify that they met the case definition used by the authors (cholecystitis with rupture; diverticular abscess; appendiceal perforation and periappendiceal abscess; acute gastric and duodenal perforation operated within 24 hours; perforation of intestines; traumatic perforation of the intestines; or intra-abdominal abscess). To obtain an estimate of the number of U.S. cases per year, we applied the rate of 2.3 cases per 10,000 person-years to the 2011 U.S. population, resulting in 72,043 cases. Subtracting the total number of deaths (14,136) from this estimate (see Section 18.104.22.168), results in around 57,489 surviving CIAI hospital patients per year.
QALYs Lost per Case
We searched the Tufts database for utility weights related to intra-abdominal infections and found reasonable QALY weights to be 0.50 for the period of hospitalization and 0.85 for the post-hospitalization period of convalescence (Richards & Hammitt, 2002). We then adjusted the utility weights by period of illness using the following equation:
The average length of stay in the hospital for complicated intra-abdominal infections is estimated in various pharmacoeconomic studies to be around 10 days (Cattan, et al., 2002; Sturkenboom, et al., 2005; Walters, Solomkin, & Paladino, 1999). To capture additional days spent sick or recovering outside the hospital, we used 21.8 days as an average length of the post-hospital convalescence period based on the average length of convalescence after appendectomies (National Center for Health Statistics, 1963). Thus, the lost QALYs per patient were estimated at 0.002266 as:
Total QALYs Lost due to Morbidity
Given that lost QALYs per CIAI case is 0.00266 and the total number of CIAI cases that do not result in death in the US is around 57,500, we computed the total annual QALYs lost due to CIAI morbidity to be 1,632 in the US.
To calculate VSLY-based illness costs (for patients who do not die), we took the VSLY for the age group containing the median age of a CIAI inpatient (age 53, VSLY of $561,250) (Sturkenboom, et al., 2005), and multiplied it by the average lost QALYs per patient (0.02266) to arrive at $12,717 per patient. The total morbidity cost due to CIAI was then computed at $731.1 million (= $12,717 × 57,489) per annum.