Measuring (1,3)-β-D-glucan in tracheal aspirate, bronchoalveolar lavage fluid, and serum for detection of suspected Candida pneumonia in immunocompromised and critically ill patients: a prospective observational study

Candida pneumonia (CP) is life-threatening and has been associated with a high attributable mortality [1]. However, the definitive diagnosis is rarely established before overwhelming sepsis or death. Conventionally, a lung biopsy has been proposed to confirm CP [2, 3], but the procedure is too risky for patients in intensive care units (ICUs) because of the high prevalence of thrombocytopenia and coagulopathy [4]. Growth of respiratory Candida spp. in immunocompromised, cancer-afflicted, and critically ill patients is frequently found, but it is usually considered colonization rather than CP [5]. Indeed, growth of respiratory Candida spp. lacks specificity to diagnose CP [6‐9], and adds little value to optimizing CP management [10, 11]. Recently, growing evidence has challenged the dogma that respiratory Candida spp. acts as a bystander in ICU patients [12]. The presence of respiratory Candida spp. has been shown to be associated with increased bacterial pneumonia development [13‐16], selection of multidrug-resistant bacteria [17], and worse ICU outcomes [14, 18‐20]. Accordingly, measurements of biomarkers with diagnostic value for the early detection of suspected CP are urgently needed.

Measurements of serum (1,3)-β-D-glucan (BDG), the common fungal wall antigen, can now be accomplished within hours. Measuring serum BDG has emerged as a rapid assessment to facilitate the diagnosis of invasive fungal infection, and it has proven to provide indirect mycological evidence for invasive fungal infection by a task force consensus [3]. A recent meta-analysis of 2979 patients included from 16 studies showed that using serum BDG levels to diagnose invasive fungal infection could attain a pooled sensitivity and specificity of 76.8 and 85.3%, respectively, and an area under the receiver operating characteristic curve (AUROC) of 0.89 [20]. However, none of these studies focused on respiratory Candida spp. infection. Additionally, it is conceivable that measuring BDG levels in respiratory specimens might be more accurate and specific for the diagnosis of pulmonary infection compared to measuring BDG levels in serum. Thus, the diagnostic value of measuring BDG levels in serum or respiratory specimens to detect suspected CP remains unclear.

In this study, we conducted a prospective, observational study aiming to compare the diagnostic value of measuring BDG levels in endotracheal aspirate (TA), bronchoalveolar lavage (BAL) fluid, and serum for the detection of suspected CP in immunocompromised and critically ill patients.

To our knowledge, the present study was the first to compare the diagnostic value of measuring BDG levels in TA, BAL, and serum for the detection of suspected CP in immunocompromised and critically ill patients. The major findings were that measuring BDG levels in respiratory specimens, including both TA and BAL fluid, exerted good diagnostic value for the detection of suspected CP, particularly in the absence of concurrent candidemia. In contrast, measuring the serum BDG level had no diagnostic value for this detection.

There are limited data regarding measuring BAL BDG levels to diagnose fungal pneumonia. The performance of BAL BDG at different cutoff value to diagnose non-Candida fungal pneumonia varied widely, in terms of sensitivity (53-90%) and specificity (26-88%) in different patient populations [25‐28]. Among these, the largest study (268 BALs) reported by Prattes et al., who compared BAL galactomannan and BDG to diagnose proven/probable invasive pulmonary aspergillosis. They concluded that despite similar sensitivity (> 90%) and NPV (> 90%) between galactomannan and BDG, BAL BDG was less convincing due to low specificity (< 50%). This might be ascribed to frequent respiratory colonization of Candida spp. [28], which potentially limited the diagnostic performance of BAL BDG for non-Candida fungal pneumonia. In contrast, we excluded most of the possible confounding factors and focused on Candida pathogens. Thus, our data showed good performance (both sensitivity and specificity >85%) by means of measuring BAL BDG. Moreover, consistent with the observation reported by Reischies et al. [29], our data also showed that Candida culture-positive specimens had higher BDG levels (Fig. 4a), which implicates that BDG levels reflect fungal burdens indirectly. Patients with higher fungal burdens are more susceptible to fungal infection. This potentially provide the rationale to measure respiratory BDG levels as good diagnostic aid to detect suspected CP.

We further showed that the TA BDG level was positively correlated with BAL BDG (Fig. 4b). Despite with inferior value to BAL BDG, measuring TA BDG also offered good diagnostic performance for the detection of suspected CP (Fig. 5). Thus, measuring TA BDG could serve as a simple, less-invasive, alternative diagnostic for suspected CP when BAL is unavailable. Previously, most cases of CP were considered to indicate lung involvement of disseminated candidiasis rather than primary CP [1, 5]. Based on this belief, patients with CP are supposed to have as high of a serum BDG level as those with candidemia. However, our data did not correspond to this notion. Patients with suspected CP had apparently low BDG in serum (vs. those with non-CP/candidemia) and high BDG in TA and BAL (Fig. 3). These findings suggested that serum BDG might have no diagnostic value in patients with suspected CP in the absence of concurrent candidemia. Thus, we suggest clinicians should directly measure BDG in respiratory specimens in patients with suspicion of CP.

To date, there have been no universally acceptable criteria to diagnose CP, and most diagnoses have depended on lung biopsies. CP is considered uncommon based on earlier lung autopsy reports [1, 6, 8], so the clinical practice guideline does not recommend using antifungal treatment based upon the Candida culture alone [3, 5, 30]. However, CP might possibly be under-diagnosed in critically ill patients because a lung biopsy is rarely performed. Indeed, even for immunocompetent ICU patients with Candida-positive respiratory specimens, 24.2% of physicians recommended antifungal treatment in an early questionnaire surveillance regarding CP [31] and 32.9% did so in a recent prospective study concerning ICU-acquired pneumonia [32]. These reports reflected the dissociation of managements between the guideline [3, 5, 30] and real-life ICU situations remain an unresolved issue. It is logically speculated that antifungal treatment are more frequently used for immunocompromised and critically ill ICU patients, particularly while these patients were undergoing non-responding or progressive pneumonia with the presence of respiratory Candida spp. and the absence of the other causative pathogens after thorough work-ups. Thus, applying antifungal treatment response in these situations might be reasonably therapeutic diagnostics for suspected CP and could compensate the gap between the guideline and clinical scenario. Accordingly, more practical diagnostic criteria for detecting suspected CP are urgently needed. Our findings showed measuring respiratory BDG levels might provide valuable information to enhance these diagnostic criteria.

There were some limitations of our study. First, our data were derived from selected patients to minimize confounding factors. Particularly, we excluded many patients with concurrent use of antibiotics. The concern that if concurrent use of antibiotics possibly confounded BDG assay remains a conflicting issue [33‐40]. It needs a large-scale study to validate this issue. Second, the sample size of this study was small, and future investigations with larger sample sizes will be needed to optimize the best TA or BAL BDG cutoff values. Third, our results were derived from the study of Candida spp.; the application of measuring BDG levels in other fungal pathogens requires further validation.

This study is not intended to replace lung biopsy for the diagnosis of CP, but try to use less invasive biomarker to detect suspected CP. Our results clearly showed that the BDG level in BAL or TA provided a good diagnostic value to detect suspected CP. Additionally, measuring serum BDG had no diagnostic value for detecting suspected CP in the absence of concurrent candidemia. These findings warrant a future large-scale investigation to validate the optimal cutoff value for BDG in respiratory specimens and to determine the timing to initiate antifungal treatment.