 In the Dutch Normal Pressure Hydrocephalus Study, containing 101 patients, the positive and negative predictive values of lumbar constant flow infusion were 92 and 34 for an Rout of 18 mmHgmlmin and 100 and 27 for an Rout of 24 3. External lumbar CSF drainage for 4 days resulted in positive and negative predictive values of 87 and 36 4. The predictive value of the cerebrospinal fluid tap test reported by Wikkelso et al in 1986 was 100 for a positive and 45 for a negative test 5 Another parameter of CSF dynamics is elastance that can be expressed by the pressure volume-index PVI. In a group of 30 patients with NPH high elastance, i.e low PVI was the best predictor of a marked and rapid reduction of ventricular size after shunting 6. 