[Reasons in favor of lumbar puncture diagnosis (or lavage)].

In patients with strong suspicion of SAH, CT is the initial diagnostic procedure of choice. A lumbar puncture (LP) should be done if a CT is not available. If the patient has no focal deficit or papilloedema there is a little risk in LP. When a CT is negative there can be indication to do a LP: small leaks can be overlooked by CT, and they are often important premonitory events preceding larger and severe haemorrhages. The accuracy of CT in documenting SAH diminishes after 24 hours: thereafter, diagnosis is often dependent on LP. In some cases LP can be useful because the procedure may alleviate headache and remove some blood. LP can also quantify cerebro-spinal fluid (CSF) pressure, provide a baseline for future CSF determination, and allow the study of some parameters like arachidonate metabolites, lactic acid, fibrinogen degradation products (FDP) and thrombin-antithrombin complex (TAT).