Short time to blood culture positivity in Enterococcus faecalis infective endocarditis

This study was conducted in the Region of Skåne in southern Sweden, comprising 1.3 million persons. The region is served by a single microbiological laboratory that has satellite blood culture cabinets (BACTEC FX, Becton Dickinson, Franklin Lakes, USA) in the five largest hospitals, where blood cultures are put into the cabinets at all hours. Five smaller hospitals are without blood culture cabinets. Due to probable and unpredictable delays before vials from these hospitals were incubated, cases from hospitals without blood culture cabinets were not included. Cultures were incubated for a maximum of 120 h. Species identification in positive blood cultures was performed using Microflex MALDI-TOF MS (Bruker Daltronics, Bremen, Germany), with the software FlexControl and MBT Compass 4.1 and the reference database MBT Compass Library DB-7854.

All episodes of monomicrobial EFsB in patients ≥ 18 years old between 2015 and 2018 in the five hospitals holding culture cabinets were reviewed and clinical and microbiological records were collected retrospectively. Episodes of EFsB in the same region between 2012 and 2016 have previously been analyzed to generate the DENOVA score [19]. Episodes for which the medical records or TTP were not available were excluded from the study, as were episodes where antibiotic treatment had been administered within 7 days before the first blood cultures were drawn. For each episode of bacteremia, the shortest TTP of all the blood culture vials drawn within the first 24 h was recorded.

Parameters needed for the modified Duke criteria [20], the Charlson comorbidity index [21], and the previously described score to determine the risk for IE in EFsB (DENOVA) [19] were collected, as well as demographic variables and outcome measured as mortality in-hospital, at 30 and 90 days and recurrence of monomicrobial EFsB within 180 days. An episode of EFsB started with a positive blood culture and was concluded by either at least 7 days of effective antimicrobial treatment or after 30 days. IE was defined as definite IE according to the modified Duke criteria. Thus, only non-nosocomial acquired EFsB was regarded as a major criterion [20]. Echocardiography reports that were ambiguous were reevaluated blindly by an experienced echocardiographer (PG) and classified as either IE or non-IE. Other foci of infection were defined as fulfillment of at least two of the following criteria (a) typical signs or symptoms of infection, (b) isolation of E. faecalis at the site of infection, and (c) imaging results compatible with focal infection [19]. Cases not fulfilling at least two of these criteria, or definite IE, were considered to have an unknown focus of infection. A focal infection was also considered an origin of infection if it was the likely site of entry of the bacteria into the blood stream. Site of acquisition was classified as nosocomial if the positive blood culture was drawn after > 48 h of hospitalization, and otherwise as community acquired. Previous EFsB was defined as growth of E. faecalis in a blood culture within the previous 90 days. Patients were followed for a minimum of 180 days after the episode through the medical records which are common to the entire region.

Statistical analyses were performed using SPSS Statistics version 26 (IBM) and Stata (StataCorp). TTP was dichotomized into TTP ≤ 12 or > 12 h which was close to the median and a practical cut-off. Patients were categorized into one of three age groups of similar sizes: 18–70, 71–80, and ≥ 81 years. Underlying comorbidity was categorized into three groups according to the Charlson score: 0–2, 3–4, and ≥ 5.

Median TTP and inter quartile range (IQR) was calculated according to grouping by age, gender, Charlson score, nosocomial vs. non-nosocomial acquisition, and focus of infection. The differences between groups were assessed with the Mann-Whitney or the Kruskal-Wallis test.

A cross-tabulation was made with dichotomized TTP as the outcome variable and variables that had a priori been postulated to possibly have an effect on TTP. Associations between the exposures and TTP were tested with the chi-square test. Logistic regression was used to estimate ORs for TTP ≤ 12 h, the following variables were included in the model: gender, age (grouped as 18–70, 71–80, and > 80 years of age), Charlson score (grouped as 0–2, 3–4, and ≥ 5), site of acquisition (community vs. nosocomially (positive blood culture drawn after more than 48 h of hospitalization) acquired), and site of infection (urinary tract, IE, gastro intestinal (GI) and biliary, skeletal or joint, other known focus of infection, or unknown focus of infection).

Another cross-tabulation was made with IE as the outcome, with TTP and other markers of risk as exposure variables. These variables were chosen based on prior knowledge [19]. Statistical significance tests using chi-square test was performed.

Multivariable logistic regression analysis was performed with IE as the outcome and TTP ≤ 12 h (yes/no) as the main predictor. These models were adjusted for age, gender, underlying comorbidity (Charlson’s comorbidity score), and site of acquisition. Both multivariable logistic regression analyses were first made with all episodes included and thereafter, as a sensitivity test, with only the first episode per patient included.

A ROC curve was plotted with IE as the outcome and TTP as the exposure.

TTP as a continuous exposure variable and the outcomes death in hospital, within 30 days, 90 days, and relapse of EFsB within 180 days, were cross tabulated and hypothesis testing was performed using the Mann-Whitney test.

A total of 359 patients with EFsB fulfilling the inclusion criteria were identified. Of these, 23 were excluded due to antibiotics having been administered before the first blood cultures and 13 were excluded due to unobtainable records or TTP. This left 367 episodes in 323 patients in the analysis. Three patients had four episodes, six patients had three episodes, and 23 patients had two episodes each. Median time between end of antibiotic treatment and a new EFsB was 30 days (IQR 16–72). Median age was 74 years (IQR 69–83), 72% were male, median Charlson comorbidity score was 2 (IQR 1–4), and 84% of the episodes were community acquired (Table 1).

Transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE) were performed in 90 of the 367 episodes, while TTE alone was performed in 70 episodes.

Median TTP for the entire cohort was 11.6 (IQR 9.9–14.1) h. Median TTP together with IQR in different groups which could be related to TTP is given in Table 1. The TTP was not equal between the different types of infection with IE having the lowest median TTP (p < 0.001). A cut-off of 12 h was used to tentatively separate a short TTP from a long TTP. This cut-off was close to the median, and it could be easy to apply to clinical practice. The proportion of episodes with a short TTP was calculated for variables potentially associated with TTP and the results are given in Table 2. The only variable clearly associated with a short TTP in univariate analysis was IE, OR 12.8 (95% CI 4.4–37) (Table 2). IE remained associated with a short TTP in multivariate analysis, OR 13 (95% CI 4.4–38). As a sensitivity analysis, all recurrent episodes were excluded leaving only the first episode in each patient (n = 323). The results were similar, OR of 12 (95% CI 3.5–41) for TTP ≤ 12 h in IE. No other variables were associated with TTP ≤ 12 h (see supplementary table 1).

To investigate how TTP was associated with IE, episodes fulfilling definite Duke criteria were compared with those episodes that did not. The receiver operator characteristics (ROC) curve for the ability of TTP to discriminate between episodes with and without IE is shown in Fig. 1. Area under the curve (AUC) was 0.80 (95% CI 0.74–0.86). With the cut-off used above at ≤ 12 h, the sensitivity for IE was 93% (CI 83–97) and the specificity 51% (CI 45–56). The positive likelihood ratio was 1.8 and the negative likelihood ratio was 0.15. If a cut-off at the lower quartile of the TTP distribution, ≤ 9.9 h, was chosen instead, the association to IE remained highly significant in univariate analysis using chi-square test. The sensitivity for IE decreased, while the specificity increased, see Fig. 1.

In Table 3 variables that are known or plausible markers of risk for IE are presented, with IE as the outcome. Univariate analyses within the cross table identified associations between IE and previous E. faecalis bacteremia, community acquisition, long duration of symptoms, embolizations, > 2 positive blood cultures, unknown origin of infection, heart valve disease, auscultation of murmur, and a short TTP. In multivariable logistic regression, the OR for IE in patients with TTP ≤ 12 h was 13.7 (95% CI 4.8–39.4) adjusted for age, gender, Charlson’s comorbidity score, and nosocomial acquisition.

A sensitivity analysis performed by removing all recurrent episodes from the analysis and leaving only the first episode in each patient (n = 323), showed similar results, OR for IE was 13.4 (95% CI 4.0 to 45.2) adjusted for age, gender, Charlson score, and nosocomial acquisition (see supplementary table 2 for univariate analyses).

The overall 30-day mortality was 11% (41 patients) and the TTP was similar between survivors and non-survivors (Table 4). In-hospital mortality was 14% and 180-day mortality was 24% and TTP was similar between the groups also using these alternative outcome-measures (data not shown).

A new EFsB was detected after 44 episodes. The median TTP in episodes followed by recurrence was 10.8 h (IQR 9.0–13.5), compared to 11.7 (IQR 10.0–14.1) in episodes without later recurrence. This difference was not statistically significant. Recurrences occurred in 32 patients who experienced two episodes (n = 23), three episodes (n = 6), or four episodes (n = 3), respectively. In ten patients who did not undergo TEE during the first episode, IE was recognized in a following episode of EFsB.