Patient characteristics and outcome
A total of 165 patients who tested positive for candidemia (n = 58) or bacteremia (n = 107) were included in the study. Patients with mixed yeast and bacteria blood cultures were excluded as well as those with positive yeast cultures (except for sputum) within seven days before IBC, to minimize the effect of Candida colonization. In 17 out of 58 candidemia patients, consecutive measurements of biomarkers were performed at days 3, 14 and 28 after initiation of echinocandin therapy (Fig. 1). The baseline clinical characteristics of patients are summarized in Table 1. Presence of a CVC or peripherally inserted central catheter (PICC) was associated with candidemia more often than with bacteremia (100% vs. 69%, P < 0.001), similarly to administration of antibiotics (83% vs. 42%, P < 0.001). Candidemia developed later after hospital admission than bacteremia (23 vs. 13 days, P < 0.001) (Table 1).
Table 1
Patients characteristics (n = 165) and parameters at the time of infection
| Candidemia* (n = 58; 35%) | Bacteremia** (n = 107; 65%) | P-value |
Demographic | | | |
Median age (IQR) | 54 (46–72) | 60 (46–72) | 0.668… |
Male gender | 34 (58%) | 76 (71%) | 0.116⊆ |
Mortality rate | 19 (33%) | 24 (22%) | 0.141¸ |
Clinical type of patients | n (%) | n (%) | |
Surgical | 19 (33%) | 25 (23%) | 0.193… |
Traumatic | 2 (3%) | 6 (6%) | 0.714¸ |
Medical | 37 (64%) | 76 (71%) | 0.340… |
Previous abdominal surgery | 14 (24%) | 13 (12%) | 0.047… |
Diabetes mellitus | 1 (2%) | 0 (0%) | 0.352¸ |
Inserted CVC or PICC | 58 (100%) | 74 (69%) | < 0.001… |
Chronic renal disease | 12 (21%) | 12 (11%) | 0.099… |
Corticosteroid therapy | 10 (17%) | 14 (13%) | 0.470… |
Preceding antibiotic therapy | 48 (83%) | 45 (42%) | < 0.001… |
Preceding antifungal therapy | 6 (10%) | 7 (6.5%) | 0.387… |
| median (IQR) | median (IQR) | |
SOFA | 3.5 (2–9) | 6.0 (2–8) | 0.191⊆ |
APACHE II | 12.5 (8–18) | 15.0 (10–21) | 0.200⊆ |
Time from admission to candidemia or bacteremia, days | 23.0 (18–30) | 13 (10–18) | < 0.001⊆ |
Acute kidney injury stage (serum creatinine range) | n (%) 18 (31%) | n (%) 40 (33%) | |
Stage 1 (110–170 µmol/L) | 4 (22%) | 16 (40%) | |
Stage 2 (171–299 µmol/L) | 11 (61%) | 17 (43%) | 0.362… |
Stage 3 (300->440 µmol/L) | 3 (17%) | 7 (17%) | |
Median serum creatinine, µmol/L (IQR) | 274 (178–348) | 180 (141–321) | 0.217⊆ |
Values are shown as n (%), …Pearson's chi-squared, ⊆ Mann-Whitney two-sample test, ¸Fisher's exact test |
* Candida albicans (n = 24), C. tropicalis (n = 11), C. krusei (n = 7), C. glabrata (n = 6), C. parapsilosis (n = 4), C. dubliniensis (n = 1), C. guilliermondii (n = 1), C. lusitaniae (n = 1), C. metapsilosis (n = 1), Saccharomyces cerevisiae (n = 1), Geotrichum clavatum (n = 1) |
** coagulase-negative staphylococci (n = 39), Enterococcus spp. (n = 13), Pseudomonas aeruginosa (n = 10), Klebsiella pneumoniae (n = 9), Escherichia spp. (n = 6), Acinetobacter spp. (n = 5), Propionibacterium acnes (n = 4), Enterobacter spp. (n = 3), Micrococcus luteus (n = 3), Staphylococcus aureus (n = 3), Stenotrophomonas maltophilia (n = 3), Streptococcus spp. (n = 2), Bacillus cereus (n = 2), Proteus penneri (n = 1), Serratia marcescens (n = 1), Actinomyces odontolyticus (n = 1), Burkholderia multivorans (n = 1), Corynebacterium spp. (n = 1) |
CVC, central venous catheter; PICC, peripherally inserted central catheter; IQR, interquartile range |
Psep Exhibits The Highest Predictive Value For 28-day Mortality
Despite no significant difference in mortality rate between candidemia (33%) and bacteremia (22%) patients (P = 0.141) at 28 days after therapy initiation, two-sample Wilcoxon rank-sum test analysis with four biomarkers (CRP, PCT, PSEP and BDG) in comparison with SOFA score (P = 0.012) identified significant differences between candidemia and bacteremia patients (Additional file: Table ST1). PSEP significantly correlated with PCT. In case of invasive candidiasis, multivariable logistic regression showed that PSEP (AUC = 0.74) exhibited a significantly higher predictive value for mortality than PCT (AUC = 0.31) (P = 0.013) (Additional file: Table ST2). The sensitivity and specificity of PSEP vs. PCT predictive values were 81% and 69% vs. 68% and 23%, respectively.
Serum CRP, PCT, PSEP and BDG for differentiation between Candida and bacterial sepsis
CRP, PCT, PSEP and BDG serum levels were determined in blood culture-positive patients (candidemia n = 58, bacteremia n = 107) (Fig. 1). For bacterial species identified, see the legend to Table 1. Prediction and differential diagnosis are summarized in Table 2. As CRP at a cut-off of 5 mg/mL as well as PSEP at a cut-off of 350 pg/mL exhibited 100% positivity for both candidemia and bacterial sepsis patients, no discrimination between both groups is possible. Similarly, PCT at a cut-off of 0.5 ng/L does not discriminate between both groups (P = 0.406). By contrast, serum PCT at a concentration cut-off of 3 ng/L was moderately discriminative, exhibiting 76% positivity in candidemia and 56% positivity in bacteremia patients (P = 0.012), and so was CRP at a cut-off of 130 mg/mL, detecting 68% of bacteremia patients and 38% of candidemia patients (P < 0.001). When analyzing single biomarker discriminative value, BDG already at a cut-off 80 pg/mL exhibited high discrimination between candidemia [58/58 (100%)] vs. bacteremia [19/107 (18%)] patients (P < 0.001). The median serum BDG concentration was significantly higher in the candidemia (1029 pg/mL) than in the bacteremia (35 pg/mL) group (P < 0.001). When the BDG cut-off was increased to 200 pg/mL (or 220 pg/mL), the discrimination between candidemia and bacteremia groups continued to rise but the percentage of BDG-positive candidemia patients dropped from 100–97% (or 95%). Therefore, we tested whether combination of the serum biomarkers could achieve high discrimination without reducing sensitivity to candidemia.
Table 2
Prediction and differential diagnostics of Candida and bacteria sepsis using biomarkers at different cut-off values
Assessed biomarkers (recent cut-off) | Candidemia n (%) | Bacteremia n (%) | P-value |
CRP (≥ 5 mg/mL) | 58 (100%) | 107 (100%) | 0.999… |
CRP (≥ 130 mg/mL) | 22 (38%) | 73 (68%) | < 0.001… |
PCT (> 0.5 ng/l) | 52 (90%) | 91 (85%) | 0.406… |
PCT (< 3 ng/l) * | 44 (76%) | 60 (56%) | 0.012… |
PSEP (> 350 pg/mL) | 58 (100%) | 107 (100%) | 0.999⊆ |
PSEP (> 700 pg/mL) ** | 58 (100%) | 101 (94%) | 0.091⊆ |
BDG (≥ 80 pg/mL) | 58 (100%) | 19 (18%) | < 0.001⊆ |
BDG (≥ 200 pg/mL) | 56 (97%) | 3 (3%) | < 0.001… |
Median values (conc.) | Candidemia (IQR) | Bacteremia (IQR) | P-value |
CRP (mg/L) | 104 (78–150) | 164 (101–234) | < 0.0013¸ |
PCT (ng/L) | 1.6 (0.90–2.80) | 2.4 (1.03–8.54) | 0.1053¸ |
PSEP (pg/mL) | 1784 (1203–3259) | 1963 (1313–3524) | 0.7773¸ |
BDG (pg/mL) | 1029 (500–1176) | 35 (0–73) | < 0.0013¸ |
Values are shown as n (%),…Pearson's chi-squared, ⊆Fisher's exact test, ¸Mann-Whitney two-sample test |
Positivity of CRP, PCT, PSEP and BDG biomarkers was expressed relative to two or three preselected cut-offs in accordance with * and **. |
*Mostly the concentration of PCT in invasive candidiasis is very low, in the range of 2–3 ng/L [7, 8]; therefore, concentrations < 3 ng/mL were used as positive values for high probability of Candida sepsis. |
**Acute kidney injury stage 2 can affect the diagnostic accuracy of PSEP; usually, the median is close to 700 pg/mL [19] and the lowest concentrations of PSEP in fungal sepsis are usually close to 700 pg/mL [11] |
CRP, C-reactive protein; PCT, procalcitonin; PSEP, presepsin; BDG, 1,3-β-D-glucan; conc., concentration |
In summary, the best biomarkers of candidemia were BDG ≥ 200 pg/mL (P < 0.001), CRP ≥ 130 mg/L (P < 0.001), PCT < 3 ng/L (P = 0.012) and borderline discriminative PSEP > 700 pg/mL (P = 0.091). Sensitivity, specificity, PPV and NPV are shown in Table 3. The combination of PSEP having excellent sensitivity and NPV with BDG having excellent specificity and PPV seems to be the most powerful laboratory approach to diagnosing invasive candidiasis.
Table 3
Comparison of biomarker cut-offs with the best diagnostic significance for invasive candidiasis
| CRP 130 mg/mL | PCT 0.5 ng/L | PCT 0–3 ng/L | PSEP > 700 pg/mL | BDG ≥ 200 pg/mL | BDG/PSEP* |
Sensitivity (%) 95% CI | 37.9 25.5–51.6 | 89.7 78.8–96.1 | 75.9 62.8–86.1 | 100 93.8–100 | 96.6 88.1–99.6 | 94.8 85.6–98.9 |
Specificity (%) 95% CI | 31.8 23.1–41.5 | 15.0 8.8–23.1 | 43.9 34.3–53.9 | 5.6 2.1–11.8 | 97.2 92.0–99.4 | 100 96.6–100 |
PPV (%) 95% CI | 23.2 15.1–32.9 | 36.4 28.5–44.8 | 42.3 32.7–52.4 | 36.5 29.0–44.5 | 94.9 85.9–98.9 | 100.0 93.5–100.0 |
NPV (%) 95% CI | 48.6 36.4–60.8 | 72.7 49.8–89.3 | 77.0 64.5–86.8 | 100 54.1–100 | 98.1 93.4–99.8 | 97.3 92.2–99.4 |
AUC 95% CI | 0.35 0.27–0.43 | 0.52 0.47–0.58 | 0.60 0.53–0.67 | 0.53 0.51–0.55 | 0.97 0.94–1.00 | 0.97 0.95–1.00 |
*BDG/PSEP - if both tests are positive, BDG ≥ 200pg/mL and PSEP > 700pg/mL |
Because PSEP is excreted by the kidney, its level may be increased in the presence of renal dysfunction (and reduced by hemodialysis). Therefore, acute kidney injury (AKI) can affect the diagnostic accuracy of PSEP. The usually reported median is close to 700 pg/mL [19] and the lowest concentrations of PSEP in fungal sepsis are reported close to 700 pg/mL [11]. We found that 100% of candidemia patients without AKI tested positive for serum PSEP at a cut-off value of 350 pg/mL and 100% of those with stage 2 AKI (the KDIGO criteria [20]) at a cut-off value of 700 pg/mL, the latter predominating among our AKI patients (Tables 1 and 2). PSEP exhibited greater diagnostic accuracy than positive PCT (< 3 ng/L, 76%) and positive CRP (≥ 5 mg/mL, 100%).
To differentiate between Candida spp. colonization and sepsis, we analyzed serum BDG concentrations in patients with peripheral blood culture-confirmed candidemia relative to the time sequence of blood- versus catheter-culture Candida spp. positivity (Fig. 2). The Candida culture from catheters were performed within the interval +/- 3 days along IBC at the time of CVC and PICC replacement (Additional file: Table ST3). Patients with catheter-related candidemia had significantly lower median BDG concentration (471 pg/mL, P < 0.001) than those with probable deep-seated candidiasis (1029 pg/mL) or catheter colonization confirmed following positive blood Candida culture (1203 pg/mL).
Monitoring Of Successful Echinocandin Therapy
Based on Additional file Table ST4, the most significant decrease was confirmed for PSEP (P = 0.0012) as early as 14 days after echinocandin therapy (Wilcoxon signed-rank test) (Fig. 3). By contrast, serum BDG and CRP concentrations decreased significantly as late as 28 days after echinocandin therapy initiation (P = 0.0038 and P = 0.03, respectively). PCT did not change over the 28-day period.
Proposal For The Use Of Results In Clinical Practice
Figure 4 provides a diagnostic algorithm for non-neutropenic and non-transplanted ICU patients at risk for invasive candidiasis and/or candidemia as an extension of a previously reported algorithm for ICU patients with suspected candidemia in sepsis of abdominal [26] and non-abdominal origin [4]. Our algorithm could accommodate both forms. The combination of PSEP and BDG could contribute to a many-fold increase (Table 3; SN = 94.8%, SP = 100%, PPV = 100%, NPV = 97.3% with AUC = 0.97) in the specificity of non-culture based methods in the early diagnosis of IC in the ICUs and may be helpful in cases of probable IC [12].