The analysis was performed from the perspective of a U.S.-based hospital. An impact inventory, presented in
Table 1, was used to identify the health and economic consequences to be quantified in the model. Consequently, we incorporated information from studies conducted in the United States and relied on U.S. sources to assign diagnostic and treatment costs (
30–32). Outcomes and costs were calculated from the time when a blood culture was ordered for a suspected bloodstream infection to patient discharge or death and information on mortality for the first 30 days after patient admission. Quality-adjusted life years (QALYs) were calculated for the lifetime of a patient. The patient population consisted of adult, hospital inpatients with suspected bacteremia for whom blood cultures were ordered. The analytical model was developed by using TreeAge Pro 2017 modeling software (TreeAge, Williamstown, MA).
(i) Diagnostic strategies.
mRDT was defined as any molecular method capable of providing a diagnosis in ≤24 h (
11) after a positive blood culture. Specifically, our mRDT definition included methods such as PCR, matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) analysis, peptide nucleic acid fluorescent
in situ hybridization (PNA-FISH), a blood culture nanotechnology microarray system for Gram-negative bacteria (BC-GN), and a blood culture nanotechnology microarray system for Gram-positive bacteria (BC-GP). On the other hand, conventional laboratory methods were defined as methods that rely on biochemical reactions and organism-specific characteristics, such as growth, pH changes, enzymatic reactions, and metabolic activity, to provide organism identification (
33). In this study, we also took into consideration whether the diagnostic technique was combined with an ASP.
Our decision-analytic model (
Fig. 1) assessed the cost-effectivenesses of the following 5 principal strategies: an mRDT (with or without an ASP), an mRDT with an ASP (strategy A), an mRDT without an ASP (strategy B), conventional laboratory methods with an ASP (strategy C), and conventional laboratory methods without an ASP (strategy D). Furthermore, we also assessed the cost-effectivenesses of 7 mRDT subcategories with or without an ASP depending on the availability of relevant data, as follows: PCR with an ASP (strategy A1), MALDI-TOF analysis with an ASP (strategy A2), PNA-FISH with an ASP (strategy A3), BC-GP with an ASP (strategy A4), BC-GN with an ASP (strategy A5), PCR without an ASP (strategy B1), and PNA-FISH without an ASP (strategy B2). BC-GN without an ASP, BC-GP without an ASP, MALDI-TOF analysis without an ASP, and diagnostic tests that do not rely on blood cultures were not included in our analysis, as there is a paucity of data on clinical outcomes.
(ii) Model inputs.
Model inputs, including mortality probabilities, length-of-stay estimates, and costs, were obtained from the published literature. Previous reviews have summarized studies assessing the use of mRDTs for the diagnosis of bloodstream infections (
11,
27–29). A recently published meta-analysis (
11) was used to identify studies estimating clinical outcomes among patients who underwent a diagnosis of bloodstream infection with either mRDT or conventional laboratory methods (
11). The effectiveness of each diagnostic strategy was assessed in terms of mortality risk. In turn, mortality was defined as either all-cause 30-day mortality (
17,
23,
34–42) or in-hospital mortality (
9,
18,
22,
24,
25,
43).
All studies that provided available mortality data (
9,
17–25,
34,
36–45) were included in our analysis, while studies conducted outside the United States (
46–51) or studies that included pediatric patients (
35) were excluded. Overall mortality estimates, which were then used to yield probabilities of survival, were obtained by pooling the mortality rates of the included studies with the use of a random-effects meta-analysis (Der Simonian and Laird) (
52,
53).
Furthermore, the Freeman-Tukey double-arcsine transformation (
54) was performed to facilitate the statistical weighting of extreme mortality values (close to zero or unity). A random-effects meta-analysis was also used to estimate the average length of stay associated with each strategy by pooling the respective results of individual studies (
17–19,
22,
23,
34,
37,
38,
40,
55). This method, as opposed to a fixed-effect model, was chosen as it accounts for the considerable interstudy differences and heterogeneity among the data from the included studies by using a simple noniterative method to estimate the interstudy effect variance (
53).
To estimate QALYs for the population of our study, we assumed a cohort population with characteristics similar to those of the average for our included studies. Specifically, using the median values across data from the included studies in the analysis, we assumed a population with a size of 195 patients (sample index) and a median age of 58 years that is composed of 53% males and 47% females. The pertinent life expectancy data were obtained from U.S. life expectancy tables (
56). We extracted the quality of life for the general U.S. population from the European Quality of Life-5 Dimensions (EQ-5D) index population norm data (
57). The EQ-5D index was chosen as it is recommended by guidelines (
58,
59). For the first 5 years after the initial bloodstream infection episode, patients were expected to accumulate QALYs poorly, as suggested by data in the literature (
60–62), and were assigned a quality-of-life utility score of 0.68 (
61). This was obtained from a study by Cuthbertson et al. that estimated the EQ-5D score 5 years after a severe sepsis episode. The quality-of-life value was discounted by 3.0% per year, as suggested by guidelines (
26). The mean discounted QALY value that we obtained and that was used as the basis of our analysis was 13.47 (standard deviation, 1.49).
All costs obtained from the literature were adjusted to 2016 dollars by using the personal health care (PHC) expenditure deflator from the Agency of Healthcare Research and Quality (
63) and then adjusted to 2017 dollars (fourth quarter) by using the personal consumption expenditure (PCE) price index (
64), as recommended by guidelines (
59). The gold standard of bloodstream infection diagnosis is the collection of blood cultures. For our study, the estimated base cost of a single blood culture was set at $118 ($178.05 adjusted) and was obtained from data reported by Shapiro et al. (
65). This baseline cost was multiplied by 3 for each patient, to better reflect the widely accepted approach that 2 to 3 blood cultures should be ordered in cases of suspected bacteremia (
66). Subsequently, the costs of different methods used for pathogen identification and antimicrobial susceptibility testing, after a positive blood culture, were also estimated (
29).
In particular, semiautomated systems, such as Phoenix (Becton, Dickinson), Vitek 2 (bioMérieux), and MicroScan WalkAway (Siemens), are conventionally used for this purpose (
67) and fall under our definition of conventional laboratory methods. Nonetheless, the average costs of conventional laboratory methods per pathogen identified vary widely, depending on hospital identification protocols and the specific sequence of reactions needed to identify each organism. As such, we approximated the average adjusted cost for conventional laboratory methods to be $3.97 ($5.09 adjusted) per patient, by employing data from a study by Tan et al. (conducted at The Johns Hopkins Hospital, Baltimore, MD) that reported the overall cost for the identification of the 55 most common pathogens causing bloodstream infections over a 1-year period (
30). The cost of mRDT was similarly obtained from data in the literature. The base-case cost values used were $45 ($67.90 adjusted), $80 ($99.61 adjusted), $99 ($105.89 adjusted), $82.72 ($139.05 adjusted), and $43 ($45.99 adjusted) for PCR (
68), BC-GP (
69), BC-GN (
42), PNA-FISH (
70), and MALDI-TOF analysis (
71), respectively. For the cost of ASPs, we used the cost estimate provided by Scheetz et al., namely, $125 ($179.29 adjusted) per patient (
31).
The patient hospitalization cost for each strategy was estimated by multiplying the average pooled length of stay for that strategy by the cost of hospitalization per day for general-medicine patients. Specifically, we used the most recent estimate (2015) of hospitalization costs in Rhode Island, provided by the Kaiser Family Foundation, which is $2,759 ($3,080.80 adjusted) per day (
32). This value was chosen both because it lies in the middle of the hospitalization cost range for various U.S. states and because Rhode Island is the geographic base of our group. Notably, this estimate accounts for all inpatient expenses, including the cost of antimicrobial agents (
32). Model inputs, including probability values, length-of-stay estimates, and costs, are summarized in
Table 2. In the text, costs, probabilities, and QALYs have been rounded to two decimals, and incremental cost-effectiveness ratio (ICER) values have been rounded to zero decimals.