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Does a normal CT scan rule out a subarachnoid hemorrhage

Three Part Question

[In patients presenting with a history of sudden severe headache] is [CT scanning alone as good as CT scanning plus lumbar puncture] in ruling out [subarachnoid hemorrhage]?

Clinical Scenario

A 24 year old man who has been previously well presents to the emergency department complaining of headache. He describes the headache as the worst he has ever had. It came on suddenly approximately 2 hours previously and has not resolved with paracetamol. It was so severe as to cause him to collapse when it started. He has no other neurological symptoms and clinical examination reveals no neurological signs. You are concerned that he may have had a subarachnoid hemorrhage and arrange a CT scan. The CT is reported as normal. You wonder if this rules out the diagnosis of subarachnoid hemmorhage (SAH) in your patient.

Search Strategy

Medline 1966-June 2008 including MEDLINE in progress and other non-indexed citations using the OVID interface on ATHENS
[(exp subarachnoid hemorrhage OR OR subarachnoid AND (exp cerebrospinal fluid OR spinal OR exp spinal puncture OR lumbar OR AND (exp tomography, x-ray computed OR CT] LIMIT to human, English and abstracts.

Search Outcome

260 papers of which 250 were irrelevant or insufficient quality for inclusion. The remaining 10 papers are shown below.

Relevant Paper(s)

Author, date and country Patient group Study type (level of evidence) Outcomes Key results Study Weaknesses
MacDonald A and Mendelow AD,
100 patients with diagnosis of SAH confirmed on angiograpahy in tertiary centre Retrospective chart reviewSensitivity of CT99 patients had had a CT, of these 20 were normal. Sensitivity=80% (CI=15-25%)This paper did not specifically address the original question. It is subject to referral bias as only patients in a tertiary centre were examined. The CT scanners used at this time were early models.
Van der Wee N et al,
175 consecutive patients with clinical suspicion of SAH Patients with negative CT then went on to have LP. CT was performed immedately, LP after 12 hours from headache onset Retrospective chart reviewSensitivity for CT117 patients had blood on CT. Of the other 58 patients, 2 had positive LP's. Overall sensitivity for CT = 95% (CI=94-98.8%)Not all patients had an LP If the gold standard is LP findings then some of the CT cases may represent false positives
Sames TA et al,
181 patients with SAH confirmed by LP, angiography, surgery or autopsy who had a CT prior to definitive diagnosis Only 3rd generation scanners included Retrospective chart reviewSensitivity at more than 24 hours after symptoms83.8%Retrospective design There were 349 patients meeting entry criteria but 92 sets of notes were unavailable for review
Overall sensitivity91.2% (CI=87-95%)
Sensitivity at less than 24 hours after symptoms93.1%
Sidman R et al,
140 patients with a diagnosis of non-traumatic SAH LP findings used as gold standard for diagnosis Retrospective chart reviewSensitivity of CT more than 12 after symptoms 49/60 had positive CT and positive LP (81.7% sensitivity CI 69.5-90.4%)Retrospective design
Sensitivity of CT at less than 12 hours after symptoms80/80 patients had positive CT and positive LP (100% sensitivity CI 95-100%)
Overall sensitivity11/140 (92.1% sensitivity) of patients had normal CT and positive LP.
Lachtaw RE et al,
Review article Review articleSensitivity of CTSensitivity of CT ranges from 95-98%. Sensitivity decreases with time (58% at 5 days, 50% at 1 week)Original data from studies is not presented Not a systematic review
Morgenstern LB et al,
107 patients with worst headache ever Patients with negative CT got LP. Scans were reviewed by 2 neuroradiologists blinded to the LP results. LP findings used as gold standard for diagnosis Retrospective case note and radiology reviewNumber of patients with normal CT but positive LP2 of 89 patients with normal CT had positive LP's. Sensitivity given at 97.5% (CI . 97.8% - 88.7%)Retrospective design Not all patients with positive CT had an LP performed
Boesiger B; Shiber J
Patients attending one hospital over a year period who presented with headache and had a CT scan and a lumbar puncture to rule out subarachnoid haemorrhage.Retrospective cohort study to calculate sensitivity of 5th generation CT scanners in order to rule out subarachnoid haemorrhage.Sensitivity of CT100% (6 patients out of 171 had positive CT scans).Small number of patients with the target condition.
Specificity of CT99.4% (One false positive CT scan)
O'Neil J; McLaggan S; Gibson R
Patients presenting to one Emergency Department who were sent for a CT scan due to clinical suspicion of subarachnoid haemorrhage over a year period.Retrospective cohort study.Sensitivity of CT76% (19 patients out of 25 patients that had the diagnosis of SAH)Over half the patients who had a negative CT scan did not go on to have lumbar puncture. (15% of patients who did have lumbar puncture had a positive result). CT formed part of the gold standard so cannot calculate specificity.
Byyny R; Mower W; Shum N; Gabayan G; Fang S; Baraff L
Patients who presented to a tertiary Emergency Department over a three year period who were diagnosed as having a subarachnoid haemorrhage.Retrospective review to determine the sensitivity of non-contrast CT in patients with headache in diagnosing subarachnoid haemorrhage.Sensitivity of CT in patients with SAHSensitivity 93% (95% CI 88 to 97%) - 139/149 patients with SAHGold standard included CT scans so can't calculate specificity. Patients may have had a negative CT scan and not proceeded to lumbar puncture due to contraindications or lack of consent. These patients would not have been included in this study although some may have had subarachnoid haemorrhage.
Sensitivity of CT in patients with SAH and normal mental status at time of presentationSensitivity 90% (95% CI 81 to 95%)
Perry J; Spacek A; Forbes M; Wells G; Mortensen M; Symington C; Fortin N; Stiell I
All patients ≥16y presenting to 2 tertiary care centres with non-traumatic headache and normal neurological examination who had a CT scan to rule out a subarachnoid haemorrhage and a lumbar puncture if the CT scan was normal. Carried out over 3 year periodProspective cohort study to calculate the sensitivity and specificity of CT scan ± lumbar puncture when used to rule out subarachnoid haemorrhage. Patients were followed up for a minimum of six months following their attendance. Sensitivity of CT scan90.1% (55 positive CTs out of 61 patients with SAH)60 patients out of 592 were lost to follow up although as the two hospitals involved in the study contained the only neurosurgical units in the region it is unlikely that these patients did go onto to have a SAH.
Sensitivity of CT scan ± LP100% (95% CI 94-100%) No patients out of the 531 negative patients was subsequently found to have SAH.


Emergency physicians need to know if CT is sensitive enough to rule out the diagnosis of subarachnoid bleeding in patients presenting with severe headache. SAH is an important diagnosis to make, the risk of re-bleeding is high if the initial bleed is missed and it is a condition for which treatment is possible. We must therefore err on the side of caution and seek investigations with a very high sensitivity to rule out the diagnosis. The use of LP as a gold standard in many of these studies can be questioned as it too has a false negative rate, particularly when performed soon after a bleed. The diagnosis of SAH is so important that sensitivity must approach 100% for CT to obviate the need for LP. The current trials found reveal 2 interesting facts. 1. That CT has a high sensitivity (91-98%) for detecting SAH, though this is not high enough to satisfactorily exclude SAH. 2. That the sensitivity of CT for SAH decreases with time. The sensitivity given in the more recent trials is approximately 95%. This is not high enough to rule out subarachnoid hemorrhage. It is more sensitive the earlier it is performed, this is the converse of LP. CT scans depend on the presence of blood being in sufficient concentration in the CSF. If the crucial level of concentration is not present then there will not be enough contrast with the CSF to detect the bleed even on the most advanced CT scanner. The advantage of CT is that it is quick and easy to perform, may be positive in the early stages of SAH and it may give information on the cause or size of the bleed. It may also exclude a space occupying lesion or other pathology.

Clinical Bottom Line

Patients with lone acute severe headache should have urgent CT; if this is negative then a lumbar puncture should be performed.

Level of Evidence

Level 2 - Studies considered were neither 1 or 3.


  1. MacDonald A and Mendelow AD. Xanthochromia revisited: a re-evaluation of lumbar puncture and CT scanning in the diagnosis of subarachnoid haemorrhage. J Neurol-Neurosurg-Psych 1988;51(3):342-44.
  2. Van der Wee N, Rinkel GJE, Hasan D et al. Detection of subarachnoid haemorrhage on early CT: is lumbar puncture still needed after a negative scan? J Neurol-Neurosurg-Psych 1995;58(3):357-59.
  3. Sames TA, Storrow AB, Finkelstein JA et al. Sensitivity of new-generation computed tomography in sub-arachnoid hemorrhage. Acad Emerg Med 1996;3(1):16-20.
  4. Sidman R, Connolly E, Lemke T. Subarachnoid haemorrhage: lumbar puncture is still needed when computerised tomography scan is normal. Acad Emerg Med 1996;3(9):827-31.
  5. Latchaw RE, Silva P, Falcone SF. The role of CT following aneurysmal rupture. Neuroimag Clin North Am 1997;7(4):693-708.
  6. Morgenstern LB, Luna-Gonzales H, Huber JC Jr et al. Worst headache and subarachnoid hemorrhage: prospective, modern computed tomography and spinal fluid analysis. Ann Emerg Med 1998;32(3 Pt 1):297-304.
  7. Boesiger B; Shiber J Subarachnoid Hemorrhage Diagnosis by Computed Tomography and Lumbar Puncture: Are Fifth Generation CT Scanners Better at Identifying Subarachnoid Hemorrhage The Journal of Emergency Medicine 2005; (29) No1: pp 23-27
  8. O'Neil J; McLaggan S; Gibson R Acute Headache and Subarachnoid Haemorrhage: A Retrospective Review of CT and Lumbar Puncture Findings Scottish Medical Journal 2005; 50(4): pp 151-153
  9. Byyny R; Mower W; Shum N; Gabayan G; Fang S; Baraff L Sensitivity of Noncontrast Cranial Computed Tomography for the Emergency Department Diagnosis of Subarachnoid Hemorrhage Annals of Emergency Medicine 2008; 51 (6): pp697-703
  10. Perry J; Spacek A; Forbes M; Wells G; Mortensen M; Symington C; Fortin N; Stiell I Is the Combination of Negative Computed Tomography Result and Negative Lumbar Puncture Result Sufficient to Rule Out Subarachnoid Haemorrhage? Annals of Emergency Medicine 2008; 51 (6): pp707-713