Laboratory Studies
Cutaneous lesions should be biopsied and evaluated with fungal stains and cultures.
Blood and CSF should be cultured for fungi and submitted for cryptococcal antigen testing.
Even with widespread disease, the routine laboratory tests (eg, leukocyte count, hematocrit, sedimentation rate) may yield normal results.
Evaluation of spinal fluid is essential in diagnosing CNS disease. Opening pressures should be measured at each spinal tap; elevated pressures (≥250 mm H2 O) portend a poor prognosis. Opening pressures in excess of 250 mm H2 O require drainage of CSF to reduce the pressure to 200 mm H2 O or lower. Prior to removal of CSF, CT scanning or MRI should be performed to exclude masses that could result in herniation.
CSF glucose concentrations are usually depressed, while CSF protein concentrations are usually elevated. Leukocyte counts in the CSF are 20/µL or higher, with a lymphocyte predominance. The CSF can be normal at times, as in patients with AIDS who are unable to mount an adequate inflammatory response or in persons with early infection. However, these patients often have positive results on India ink preparation and CSF cryptococcal antigen testing.
Serologic testing of blood and CSF should be done whenever cryptococcal CNS infection is considered.
Antigen tests are frequently out of reach in impoverished countries due to cost. As a result, cryptococcosis often goes undiagnosed in these areas. In 2009, a lateral flow assay (LFA) for diagnosing cryptococcosis was developed. A study of HIV-infected patients in Thailand compared LFA results with culture and enzyme immunoassay (EIA). Results showed a high level of agreement between LFA and EIA testing, which suggests LFA may be potentially useful as a point-of-care assay in resource-poor areas. [27] More recently, another study was performed at a French university hospital center to evaluate the efficacy of the LFA diagnostic technique. The study revealed an excellent negative predictive value of the cryptococcal antigen LFA. This, in concert with the rapid and ease of performing the test, suggests that it can be used as an alternative in the diagnosis of cryptococcal infection. [28]
Culturing for Cryptococcus may be appropriate, even when the CSF profile is unremarkable. In patients with an indolent waxing and waning course, CSF abnormalities may persist, indicating continued disease activity. A 2016 study has established the effectiveness of an economical and easy-to-use PCR method that uses restriction digest in delineating the two species, which would be helpful in administering appropriate management. [29]
Smear and culture
An India ink preparation is commonly used with CSF to identify the organism and to support a presumptive diagnosis. If performed correctly, 25%-50% of patients with cryptococcal meningitis show cryptococci.
Diagnosis depends on detecting the organism with culture; therefore, always confirm positive smears with cultures.
Culture centrifuged CSF specimens on 3 or more occasions to increase the yield.
Obtain urine and sputum cultures, even if renal or pulmonary disease is not clinically evident.
In patients with AIDS and with cryptococcal pneumonia, the culture sensitivity of bronchoalveolar lavage washings is better than that of transbronchial biopsy specimens.
Positive blood culture results indicate extensive infection, and the organism may be observed within peripheral leukocytes or bone marrow macrophages in these patients. Use the lysis-centrifugation method of blood culture, which is the most sensitive and rapid.
Whenever cryptococcosis occurs at any site, carefully search for lesions elsewhere, both inside and outside the CNS.
Pathogenic C neoformans can be primarily isolated by streaking clinical specimens on Sabouraud dextrose agar, with or without antibiotics for bacterial growth suppression. C neoformans grows at 37°C (98.6°F), assimilates inositol, produces urease, and does not produce mycelia on cornmeal agar. C neoformans also produces melanin when incubated on agar that contains seeds from the common weed Guizotia abyssinica.
Imaging Studies
When the clinical presentation is not that of acute pyogenic meningitis, consider obtaining a CT or MRI of the brain prior to performing a lumbar puncture. This is especially important in patients who present with focal neurologic deficits or a history compatible with slowly progressive meningitis. If a mass lesion is identified, do not perform a lumbar puncture to obtain spinal fluid; rather, consult a neurosurgeon for an alternative procedure.
Axial T2-weighted magnetic resonance image shows clustered hyperintensities in the left caudate; these are consistent with enlarged Virchow-Robin spaces caused by small cryptococcomas.
In patients who are asymptomatic and immunocompetent, radiographic findings can include patchy pneumonitis, granulomas ranging from 2-7 cm, or miliary disease similar to that observed in persons with tuberculosis.
Procedures
Because of the neurotropism of C neoformans, perform a lumbar puncture in all patients with known or suspected cryptococcal disease. Most patients do not present with a clinical picture of an acute pyogenic meningitis; thus, the patient may undergo CT scan or MRI prior to a lumbar puncture. This procedure allows the physician to detect a mass lesion that may increase the risk of CNS herniation following a lumbar puncture. Both CT scanning and MRI can also reveal the presence of hydrocephalus caused by basilar meningitis. Once invasive cryptococcal disease is confirmed, initiate effective antifungal therapy.
Histologic Findings
In spinal fluid, urine, and tissue, pathogenic strains of C neoformans grow as round-to-oval yeast, surrounded by a polysaccharide capsule composed of mannose, xylose, and glucuronic acid. The yeast may be single or may have a single budding daughter cell. Cell size varies widely and ranges from 3.5-8 µm in diameter. Rarely, pseudohyphae develop.
India ink, which outlines the organisms by negative contrast, helps to identify the yeast cells in fluids or macerated tissue samples. In fixed tissue, the capsule of C neoformans may also be stained with mucicarmine, which preferentially stains mucopolysaccharides. Tissue sections can be stained with the Fontana-Masson stain to detect melanin precursors in the yeast cell wall. The presence of melanin or melanin precursors is useful in differentiating C neoformans from other yeasts.
-
Axial T2-weighted magnetic resonance image shows clustered hyperintensities in the left caudate; these are consistent with enlarged Virchow-Robin spaces caused by small cryptococcomas.
