Table 4 Characteristics of the main microbiological tests for IFI in ICU

•Gold standard for diagnosis of invasive fungal infection

•Performed on various samples (blood, sputum, BALF, CSF…) with different level of sensitivity

• Allow the execution of susceptibility test

• Direct test targeting specific DNA sequences of various fungal species

• Performed on various samples (blood, sputum, BALF, CSF…) with high sensitivity

• Allow testing main molecular mechanisms of antifungal resistance.

• Indirect test detecting polysaccharide antigen of Aspergillus cell wall released into the blood and other body fluids even in the early stages of fungal invasion

• Generally performed on BALF and serum

• Indirect test detecting (1→3)-β-D-glucan, a component of the cell walls of various fungi, particularly Candida species, early released into the bloodstream when fungal cells break down or during active fungal growth.

• Generally performed on serum, but also possible on other materials (i.e. BALF, peritoneal fluid)

(see clinical application)

Invasive candidiasis

Clinical application in ICU

• Diagnosis confirmation [29, 30]

• By providing identification of species and susceptibility profile, it is essential for choosing the targeted antifungal therapy and dosage [29, 31]

• Negative follow up blood culture during treatment of candidemia underline positive response to treatment and may define duration of therapy [32]

• Follow-up blood cultures after discontinuation of therapy could reveal any potential relapse of candidemia, especially in high-risk patients [29]

Not commonly used

Not used

• If candidemia is suspected, serum BDG may provide earlier diagnosis than traditional culture methods [33]

• Its high NPV allow early interruption of an empirical antifungal therapy in case of negative result [34]

• Serial dosing of BDG level may appraise of the efficacy of antifungal therapy and guide treatment duration [35]

• higher BDG value at diagnosis is associated with a higher risk of therapeutic failure [35]

•.Mannan Antigen (Mn): cell wall component of Candida spp, Combination of Mn and anti-mannan antibody (Mn/A-Mn) testing demonstrated high sensitivity and specificity in IC diagnosis [36]

• Candida albicans germ tube antibody (CAGTA): detection og IgG antibodies against several superficial antigens of the germ tubes of C. albicans by indirect immunofluorescence. Higher performance when used in combination with other biomarkers of IC (i.e BDG) [37, 38]

Limits

• High turn-around time (48–72 h up to 8 days) may affect early diagnosis and treatment [29, 39]

• sensitivity may be influenced by various factors (e.g. prior therapy, volume of sample cultured, timing of sample collection from the onset of symptoms) [29, 39]

-

-

• Not suitable for early identification of patients at risk of IC due to possible false positive results, (i.e. in presence of heavy mucosal or skin fungal colonization, albumin and immunoglobulin administrations, haemodialysis, packing with surgical gauzes, non-glucan-free laboratory equipment [40]

• Possible false negative results with some Candida species (e.g N. glabratus) [40]

• Lower sensitivity in candidiasis than candidemia [29]

• BDG guided antifungal treatment did not demonstrated to improve survival among ICU sepsis patients with risk factors for IC [41]

Invasive pulmonary aspergillosis

Clinical application in ICU

• Diagnosis of proven infection on biopsy (culture + microscopy)[39, 42]

• Diagnosis of possible infection on BALF/tracheal aspirate [42]

• By providing identification of species and susceptibility profile, it is essential for choosing the targeted antifungal therapy and dosage [43]

• Monitoring treatment response by evidencing changes in the growth pattern or susceptibility profile [44]

• Diagnosis of proven infection on biopsy (PCR +microscopy) [45, 46]

• PCR on BALF may be a complementary tool for early diagnosis of IPA [42, 45]

• Rapid identification of triazole resistance [47]

• Diagnosis of possible infection on BALF or serum [43]

• High sensitivity on BALF allows early diagnosis in patients with consistent clinical and radiological signs [42, 43, 48, 49]

• Serial quantitative GM levels in BALF may be valuable for assessing the response to therapy [43, 50, 51]

• Both BALF and serum BDG may contribute to the microbiological diagnosis of pulmonary aspergillosis [39]

• Aspergillus-specific lateral-flow device (LFD): rapid diagnostic test based on monoclonal antibody able to detect an extracellular mannoprotein antigen of Aspergillus species. May be performed on serum or BALF [52]

Limits

• Slow turn-around time, up to several days [53]

• Susceptibility test is generally available in few Centers [53]

• False-positive results in case of colonizing or non-vital organisms [45, 54]

• Possible false positive results with some medications (e.g., piperacillin-tazobactam) and other fungal infections (e.g. talaromycosis); the result needs carefully interpretation along with clinical and radiological features [55]

• Controversial association between GM level and response to therapy [51]

• Quantitative serum GM testing demonstrated reduced sensitivity compared with GM in BALF [50]

• Possible false positive results [53]

Other fungal infections

Clinical application in ICU

• Diagnosis confirmation in case of mold infections, species identification, and possibly susceptibility test [13]

• Identification of rare fungal species, especially slow-growth fungi [13, 56]

• Earlier diagnosis than traditional culture methods [56]

• Useful in case of deep-seated infections (CNS Infections) [56, 57]

Not conventionally used

• Serum BDG is commonly used for diagnosis of Pneumocystis jirovecii pneumonia in immunocompromised patients [58]

• Negative result of serum BDG excludes diagnosis of PJP [58]

• Cryptococcal polysaccharide antigen on serum or CSF or Histoplasma antigen on urine demonstrated good sensitivity and specificity in diagnosis of disseminated disease in immunocompromised hosts [59,60,61]

Limits

• Low diagnostic yield, especially on blood [13]

• High turn-around time[13]

• Susceptibility test is available in few Centers[13]

• Possible false positive results (e.g. colonization) [62]

• Possible false negative results [13]

• Available in few Centers [62]

• Lack of standardization of tests [62]

• Possible false positive results [58]