Insertion of an external ventricular drain (EVD) is a common neurosurgical procedure, for example in patients with intracranial bleeding, head trauma of tumours. The aim of an external ventricular drain is either to drain cerebrospinal fluid (CSF) or to measure intracranial pressure. In patients with external ventricular drains, it is a common practice in many centres, including UZ Leuven, to routinely sample CSF two or three times a week. The following tests are routinely performed: protein level, glucose level, lactate level, white cell count, red cell count, and cultures. Gram stain is performed in case white blood cell count exceeds 5 cells/mm³. This regular sampling (1) permits early detection of ventriculitis, which may be rather asymptomatic, especially in sedated patients,and (2) may guide neurosurgeons to the ideal timeframe to either taper the external ventricular drainage, or replace it by a ventriculoperitoneal shunt.

A typical external ventricular drainage system consists of a ventricular catheter, which is nowadays usually impregnated with antibiotics (e.g. Codman Bactiseal - coated with clindamycin and rifampicin), an extension tube, a drip chamber, and a collection bag. Multiple three-way taps and access ports along its course allow for CSF sampling (or injection of therapeutics, such as tissue plasminogen activator, or saline to flush the catheter in case of blockage).

From a practical perspective, CSF sampling from the external ventricular drain is possible at all of these three-way taps and access ports. Broadly, one may sample CSF as proximally (close to the ventricle) as possible (e.g. by using the access port), or distally (at the level of the drip chamber, e.g. by using the three-way tap between the drip chamber and the collection bag). There is no consistency in for CSF collection site amongst different institutions. Traditionally, in UZ Leuven, we have been sampling CSF proximally until now.

The main theoretical advantage of proximal sampling is that actively aspirated "fresh" CSF close to the ventricle likely closely resembles the true characteristics of our study object, i.e. intraventricular CSF. Nevertheless, this approach is not without risk. First, it requires to directly access the sterile inside of the extension tube. This inherently comes with a risk of iatrogenic infection. Secondly, by aspirating the external ventricular catheter, the neurosurgeon may induce an iatrogenic haemorrhage in the brain parenchyma or the ventricle, and/or direct trauma to the brain parenchyma, as in suboptimally positioned catheters or small ventricles, white matter or choroid plexus may be aspirated along with CSF. Because of the risk of iatrogenic infections, haemorrhage or trauma, it is our institutional policy that CSF sampling from external ventricular drains is performed by neurosurgeons only. This also poses practical problems and may delay sampling, especially in patients who are not admitted to neurosurgical wards (e.g. paediatric cases).

From a theoretical perspective, distal CSF sampling, at the level of the drip chamber, has multiple advantages. First, the risk of iatrogenic infections induced by CSF sampling is likely to be lower, as there is no direct contact between the CSF in the drip chamber and the intraventricular CSF. Secondly, there is no risk of inducing haemorrhages or brain trauma, as no "active" aspiration is required for CSF sampling. Hence, distal CSF sampling can perhaps also be carried out by physicians from other fields than neurosurgery, or by trained nurses. However, theoretically, it is possible that the characteristics of CSF sampled distally do not sufficiently resemble those of intraventricular CSF.