Prior Candida spp. colonization is an independent risk factor, particularly in patients with multifocal fungal colonization [13]. According to León et al. [13], mortality rate was higher in patients with multifocal colonization, with 50.9% against 26.5% mortality rate in patients with unifocal colonization. Multifocal colonization was defined when Candida spp. were simultaneously isolated from various non-contiguous foci [13]. A previous study did not associate colonization with infection risk [14], yet only rectal and/or urine isolates were collected. Pittet et al. [15] demonstrated a higher risk of fungal infection depending on the intensity of colonization—patients colonized at more than two sites.

Etiology has seen major shifts through time towards non-albicans Candida, and different Candida spp. identification are related to different contributing factors. Candidemia by C. glabrata is described in patients with solid organ transplants and with previous antifungal therapy [10,16,17]. C. parapsilosis is identified in patients with recent surgery, patients with intravascular devices or parental nutrition [10,18]. C. parapsilosis has a particular affinity to intravascular devices due to their adherence ability and biofilm formation [19].

C. tropicalis and C. krusei were mainly isolated in patients with hematologic malignancies [10,16,17]. Patients on dialysis or with HIV infection were prone to Candida dubliniensis infection [10]. Candida guilliermondii cases had prior antifungal exposure [10]. Candida lusitaniae had solid tumor history and recent surgery [10]. Candida auris is an emerging multidrug-resistant yeast in patients with surgery or intravascular devices and with previous antifungal therapy [20] and is prone to cause nosocomial outbreaks [21]. C. albicans infection has a lower mortality risk when compared to non-albicans Candida [12,22].

The emergence of resistance in Candida spp. has raised concern in critically ill patients and threatens appropriate antifungal therapy. C. glabrata is the most commonly resistant identified species [12]. The reduced susceptibility to azoles has modified antifungal prescription practices for echinocandins; as a result, selection pressure increased resistance to echinocandins [23,24,25].

The base of the immune response to invasive candidiasis is a neutrophil function, and neutropenia is a well-recognized risk factor for invasive fungal infection. Monocytes/macrophages also play an important role in protection. Cancer patients have immune defects and a disruption in intestinal mucosal integrity that allows local Candida spp. overgrowth and access to the bloodstream. Patients with candidemia have a 30 to 50% cancer prevalence [10,26]. There are a number of cancer diagnoses related to candidemia, such as acute leukemia, lymphoma or myelodysplastic syndrome, allogeneic hematopoietic cell transplantation and graft versus host disease [26,27,28].

Among solid tumors, the risk correlates with Candida colonization sites. Patients with gastrointestinal cancer have the majority of cases of invasive fungal infections, followed by genito-urinary cancer [26,27].

Bloodstream infections caused by Candida spp. are mainly caused through the intestinal barrier, particularly relevant in patients after surgery due to mucosal damage.

The incidence of candidaemia is higher on surgical ICUs when compared to medical ICUs [14,22]. Surgery is a major risk factor and is well-proven when involving the gastrointestinal tract [14,26]. This association is explained by Candida spp. colonization in this site. Patients submitted to upper gastrointestinal tract surgery or the presence of gastroesophageal junction leakage is a risk factor for Candida infections [29]. Patients under thoracic surgery also had a risk for candidaemia, while trauma and neurosurgical cases had a lower risk [14].

Surgical patients usually recover with hyperalimentation fluids, a relevant risk for candidemia. Total parental nutrition has been associated with a higher risk for candidaemia than peripheral parental nutrition (OR 26.8 vs. 20.0) [30].

Catheter-related Bloodstream Infection (CRBSI) is defined as occurring 48 h before or after catheter removal and positive culture with the same microorganism of either quantitative CVC culture ≥ 10³ CFU/mL or semi-quantitative CVC culture > 15 CFU or BSI occurring with or without catheter removal, and quantitative blood culture ratio CVC blood sample/peripheral blood sample > 5 or differential delay of positivity of blood cultures (CVC blood sample culture positive two hours or more before peripheral blood culture) or positive culture with the same microorganism from pus from insertion site [31].

Skin colonization is the first step for invasive candidiasis. Devices disrupt the physical barrier of the skin and mucous membranes allowing the fungus to access the blood. Cardiovascular invasive procedures and the presence of intravascular catheters are common risk factors [10,32].

Patients admitted to intensive care units have the highest risk of healthcare-associated infection (HAI), 19.2% compared to 5.2% on the 2018 European point-prevalence survey (PPS) [1]. Bloodstream infections were the fourth most frequently reported HAI, 10.8% in PPS [1]. Candida spp. was one of the 10 most frequently isolated microorganisms [7]. In ICU, BSIs are the third most common site of infection and the highest infection-associated mortality [3]. Among patients with positive microbiological cultures, 16% had a fungal microorganism [3].

Nosocomial BSIs often are related to the presence of a catheter; therefore, ICUs have higher rates of catheter-related BSI.

Septic shock in the setting of candidemia was believed to occur less than bacteremia septic shock, but in a 2016 published EUROBACT study [32], 39.6% of patients admitted with fungemia presented with septic shock against 21.6% of bacteremia patients. Candida septic shock carries a high crude mortality, reported being in the range of 36 to 61% [3,33,34,35]. Risk factors associated with higher mortality in ICU are failure of source control, delay in antifungal therapy, and increasing APACHE score [35]. Patients with vasopressors who underwent renal replacement therapy or positive ventilatory support present an increase in the volume of distribution and are exposed to antifungal underdosing, and therapeutic drug measurement (TDM) is advised.

Antibiotics are responsible for changes in endogenous microbial flora, which allows fungal overgrowth on site. The use of broad-spectrum antibiotics is believed to be a risk factor for Candida spp. infection [36], particularly when more than two drugs are used [11]. In a recent retrospective study on catheter-related C. parapsilosis BSI, patients with prior use of more than three antibiotics had seven times greater risk of candidemia [37]. Quinolones and third generation cephalosporins were the mainly used antibiotics associated with Candida BSI in intensive care units [38].

Antibiotic consumption has decreased between 2019 and 2020 (consequences of the SARS-CoV-2 pandemic), yet antibiotic consumption is higher in critical care compared to infirmary patients. On the other hand, antifungal consumption has increased, possibly due to an increase in fungal co-infection in patients with COVID-19 and corticosteroid use [39]. Previous use of antimicrobial therapy must be evaluated in all patients with suspected Candida spp. infection.

The standard of care for definite diagnosis is the isolation of Candida in blood cultures (BC), sill it is not an early diagnostic technique. Sensitivity of BC to detect Candida ranges from 50 to 71%. Still, it can be lower in neutropenic patients [43,44]. Candida isolation can take between two and three (some until eight) days to grow [45]. Time to positivity is different between species. C. glabrata grows slower than C. albicans [46]. Blood cultures can be negative in patients with antifungal drug exposure, and sensitivity can be increased when the volume of the complete set of blood cultures is 60 mL. In patients with probable invasive candidiasis, the recommended frequency of blood culture collection is daily [44]. Non-culture diagnostic tools do not substitute blood cultures. They can only be combined for earlier intervention. After organism identification in BC, an antifungal susceptibility test is required to guide the management of candidaemia and oral azole de-escalation due to the emergence of resistance to azoles and echinocandins, significant in C. glabrata.

Biomarkers are essential tools for early diagnosis; still, despite extensive research, they are not validated to distinguish colonized patients from patients with fungal infections. Available biomarkers, such as mannan, anti-mannan antibody and 1,3-β-D-glucan (BDG), have been developed to improve and anticipate the detection of invasive disease prior to microbiological confirmation.

Mannans are a main cell wall component of Candida spp. and are used to detect Candida infections [47]. The combined detection of mannan and anti-mannan antibody increases the sensitivity from 58% to 83% and specificity from 59% to 86% [48,49]. In the ICU setting, they have a high negative predictive value, which is particularly useful in excluding invasive Candida infections, especially after five days of unnecessary antifungal therapy [33]. These biomarkers can predict infection prior to blood cultures. They are an important tool in reducing the diagnosis time yield or reducing the use of antifungal agents. For non-albicans bloodstream infections, such as C. parapsilosis and C. krusei, antigen and antibody detection have lower sensitivity [50].

BDG is a pan-fungal diagnostic test. BDG is a cell wall component of Candida and other fungi (such as Aspergillus, Pneumocystis jiroveci and others) with a high diagnostic sensitivity of 75–80% and specificity of around 80% [49] Odabasi et al. [51] reported positive BDG result up to 10 days before clinical diagnosis in patients with proven or probable invasive fungal infection.

To optimize BDG performance, two consecutive positive results are required. A meta-analysis to evaluate the accuracy of BDG on ICU patients by Haydour et al. [52] reported 80% of sensitivity but low specificity (only 60%). Patients with albumin, renal replacement therapies with cellulose membranes, intravenous immunoglobulin and concomitant BSI, may have false positive BDG [43]. BDG can be used to withdraw unneeded antifungals due to high negative predictive value (NPV) in ICU [53,54].

Polymerase Chain Reaction (PCR) performed in blood samples have the highest sensitivity, 90–95%, and specificity, 90–92%. PCR shortens the time to a diagnosis, yet the interpretation is heterogenic, and colonized patients may have a positive PCR [55,62]. The need to follow a strict aseptic technique to obtain the blood for this test is as important, if not more, than for the routine blood cultures because PCR detects very small quantities of the genetic material of either viable or non-viable microorganisms.

The detection limit of PCR is under 10 CFU/mL; still, if the number of Candida CFU/mL in the blood is under the threshold, like in an early set of the disease, the test might be negative. Pfeiffer et al. [45] reported CFU/mL of ≤1 on half of Candida spp. blood cultures, particularly in patients with candidemia by C. glabrata. Low organism burden was associated with neutropenic patients, recent major surgery, end-stage live disease, renal replacement therapy, interrupted gastrointestinal tract and candidemia from the abdominal site [45]. The five most common pathogenic Candida spp., such as C. albicans, C. glabrata, C. parapsilosis, C. tropicalis and C. krusei, can be targeted by commercial multiplex PCR kits. Specificity is superior in molecular amplification techniques over BDG and CAGTA, and, as a consequence, better positive predictive values can be achieved [55].

The BioFireFilmArray BCID assay identifies 24 organisms (19 bacteria and 5 most common Candida species) by multiplex PCR from positive blood cultures [63], with a sensitivity of 100% and results in one hour [64]. An updated version of the panel—BCID2 identifies 33 species, including 7 fungi, 6 Candida spp. (C. albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. krusei and C. auris) [65].

A novel nanodiagnostic test, T2Candida Panel, is a PCR-based assay that detects Candida within whole blood through mechanical lyses of cells and DNA amplification later detected by amplicon-induced agglomeration of super magnetic particles and T2 magnetic resonance measurement. Sensitivity and specificity are 89–91% and 98–99% [66,67]. The limit of detection is 1 CFU/mL [67], and the limit of blood cultures is 1 CFU/60 mL of blood, usually obtained in a routine set of three 20 mL samples. An important feature is the availability of the result in 3–4 h directedly from whole blood. The T2Candida Panel reports the five most common Candida spp. as a positive or negative result. Results are reported based on susceptibility to fluconazole and divided as C. albicans/C. tropicalis, C. parapsilosis, and C. krusei/C. glabrata. An important benefit of this novel test is the higher sensitivity on follow-up analysis versus blood cultures, demonstrated in already Candida BSI patients either with neutropenia or in patients receiving prior antifungal therapy [68], probably because it is a genetic amplification assay that detects non-viable yeasts. Positive blood cultures and positive T2Candida in candidemia follow-up samples had higher mortality (42% vs. 5% when they were negative) [68].

All these molecular tests do not discriminate colonization or past infection from ongoing true infection, and diagnostic stewardship is advised to avoid over-diagnosis of true active fungal infections.