Invasive fungal infections are a particular risk for those with impaired immune systems. The period of risk can be short, such as those with low white cell counts for a few days or weeks following chemotherapy. In others, the risk period is long and is determined by how easily the immune deficit can be reversed – for example, immediately following renal transplantation, when the immune system is markedly depressed to prevent rejection of the new kidney. As time passes, anti-rejection drug doses are gradually reduced, thereby reducing the risk of infection. They are increased again if rejection occurs. If a life-threatening fungal infection does occur, one option to preserve life is to completely stop the anti-rejection drugs, sacrificing the new transplanted kidney (rejection is almost inevitable), but enabling the person to survive the fungal infection.
In general, an overall assessment of immune status is done by experienced clinicians who calculate the ‘net state of immunosuppression’. This assessment guides diagnostic and empirical antimicrobial choices, and is combined with prior test results, drug tolerability and interactions, as well as environmental factors. Clinician experience, combined with precise, rapid diagnostic testing, is the most important factor determining survival from invasive fungal infection (Brown et al, 2012).
As the whole immune system is complex and functions as a network, few straightforward tests are available to get a precise handle on immune function at any one time.
- In HIV infection the CD4 cell count is an imprecise but useful general measure of immune status.
- In leukaemia patients, the neutrophils (granulocyte) count is an approximate measure of risk.
- In people taking corticosteroids (glucocorticoids), the dose and duration are approximate measures of risk and survivability from an invasive fungal infection.
- Following a significant bacterial infection, surgery or trauma, the immune system naturally goes from activation to depression (so called immunoparalysis) so that the immune reaction itself is not fatal. This period of immunoparalysis is a high-risk period for fungal infections.