Monitoring central nervous system methotrexate levels via subcutaneous reservoir. Sources of errors in sampling and...

The Ommaya reservoir allows for ready, well-tolerated direct access to the intraventricular cerebrospinal fluid (CSF). The toxicities associated with central nervous system (CNS) methotrexate (MTX) therapy have been attributed to prolonged high levels of the drug. When samples of CSF are removed from a reservoir for the monitoring of MTX levels, there may be sampling errors introduced by retention of high concentrations of drug in the reservoir. Using an in vitro system we confirm this possibility and show that using different injection or sampling techniques will increase or decrease this error. Without proper sampling, particularly several days after the initial injection, one may overestimate the MTX level and may unnecessarily alter treatment. A simple method of avoiding this error is to flush the reservoir after drug injection with a small (3-5 ml) volume of CSF withdrawn prior to injection.