Objectives Acute hydrocephalus ensues from various intracranial processes and is usually treated using external ventricular drainage (EVD). After the acute phase, a clamp trial is usually performed to identify patients requiring permanent CSF shunting. The aim of our study was to identify simple and clinically accessible factors that can help predict the result of the EVD clamp trial. Methods 86 patients were selected for this retrospective analysis. Average CSF drained volume over 3 days given a constant EVD pressure level of 15 cm H₂O and other clinical and laboratory parameters were statistically compared with the result of an EVD clamp trial. Results The univariate analysis identified significant differences between the groups of patients who failed or passed the EVD clamp trial for the mean daily drained volume over 3 days (Mann-Whitney U-test, p < 0.01). In the multivariate logistic regression, the average daily CSF output over 3 days was also significant (p = 0.02), no other significant factors could be identified. Using hierarchical clustering, the best threshold daily value for EVD clamp trial failure was found at 130 ml (mean daily drained CSF volume) with the sensitivity of 64.5% and specificity of 83.6% (chi-square 20.6, p < 0.01). Conclusion We have identified a simple, clinically available factor for identifying patients who are likely to fail the EVD clamp trial.