Serum glucan levels are not specific for presence of fungal infections in intensive care unit patients

Abstract

Fungal infections in the critically ill patient are difficult to diagnose and are associated with a high mortality rate. A major obstacle to managing fungal infection is the lack of a reliable clinical assay that will rapidly identify patients with fungal sepsis. Glucans are polymers of glucose that are found in the cell wall of fungi and certain bacteria. Glucans are also released from the fungal cell wall into the extracellular milieu. Several studies have reported that detection of fungal glucan in serum or plasma is useful in the diagnosis of mycoses. However, recent studies have questioned the clinical utility of this assay. In this study, we examined serum glucan levels in intensive care unit (ICU) patients and attempt to correlate serum glucan levels with the presence of fungal infection. Following attainment of informed consent, serum was harvested from 46 ICU patients with confirmed fungal infections, confirmed bacterial infections, or no evidence of infection. Sera from eight healthy volunteers served as control. Serum glucan was assayed with a glucan-specific Limulus assay. Serum glucan levels were increased (69.6 +/- 17 pg/ml; P < 0.001) in ICU patients versus the normal (11.5 +/- 1.3 pg/ml) and noninfected ICU (27.4 +/- 17 pg/ml) controls. However, serum glucan levels were not different in patients with confirmed fungal infections versus those with confirmed bacterial infections. Thus, serum glucan levels did not show a correlation with the presence of fungal infections and do not appear to be specific for fungal infections. However, the assay may be useful as a negative predictor of infection.

FIG. 1.

Diagram of the (1→3)-β-d-glucan and LPS Limulus assay. The assay is based on the ability of L. polyphemus hemolymph to clot in response to glucan or LPS. Upon contact with (1→3)-β-d-glucan or LPS, Limulus amebocytes degranulate and release zymogens that become active serine proteases. Glucan is recognized by Factor G, while LPS is recognized by Factor C. The G test is specific for (1→3)-β-d-glucan because the amebocyte lysate is processed to remove Factor C, thus making the lysate specific for the glucan pathway. In the assay, the clotting enzyme cleaves pNA from the chromogenic substrate peptide. The free pNA is measured at 405 nm in a kinetic microplate assay.

FIG. 2.

Serum glucan levels were significantly increased in ICU patients with confirmed infections compared to levels in normal controls and ICU patients without infections. However, assaying serum glucan levels did not appear to be specific for the presence of fungal infections, since glucan levels were increased in ICU patients with either fungal or bacterial infections. A 20-pg/ml cutoff was established based on the normal control values. The data are presented as individual patient values. The bars show the mean value for each group.

FIG. 3.

A positive blood or pulmonary culture in ICU patients with fungal or bacterial infections did not correlate with serum glucan levels. Serum glucan levels in ICU patients with no evidence of fungemia, bacteremia, or pulmonary infection were equivalent to levels in ICU patients with positive blood cultures. The data are presented as individual patient values. The bars show the mean value for each group.