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Research

Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study

BMJ 2015; 350 doi: https://doi.org/10.1136/bmj.h568 (Published 18 February 2015) Cite this as: BMJ 2015;350:h568

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Re: Differentiation between traumatic tap and aneurysmal subarachnoid hemorrhage: prospective cohort study

Dear Editor,

Aneurysmal subarachnoid haemorrhage (SAH) can have serious consequences for a patient if the diagnosis is missed. This is more likely to occur if the CT scan is normal. In these patients it is common practice to perform a lumbar puncture (LP) to rule in or rule out a SAH. Therefore I read with interest this research paper by Perry JJ, Alyahya B, Sivilotti MLA et al.1

A few important methodological issues are of sufficient magnitute that they warrant mention. The sensitivity of red cell count < 2000×106/L, on its own, for excluding SAH was 93%. 95% confidence intervals ranged from 66% to 99.7% demonstrating that these results were not very precise.

On average only four patients per department per month for ten years was eligible for inclusion and only 30% were actually enrolled. 41% were excluded because they did not have a lumbar puncture and 29% due to incomplete data. The type of missing data and reasons for it are not discussed.

The presence of xanthochromia was assessed visually in 5 of 6 sites and by spectrophotometry in the other. All ssamples in each site should have been assessed using the same method. The gold standard for diagnosis of xanthochromia is subject to much debate2. In the remaining 6 sites the method used is not stated.

There does not appear to have been any follow-up of those whose CT was normal and therefore it is possible that SAHs were missed which might have influenced the sensitivities of the LP and xanthochromia. The results should be interpreted with caution.

1. Perry JJ, Alyahya B, Sivilotti MLA et al. Differentiation between traumatic tap and aneurysmal subarachnoid haemorrhage: prospective cohort study. BMJ 2015;350:h568.
2. Arora S, Swadron SP, Dissanayake V. Evaluating the Sensitivity of Visual xanthochromia in Patients with Subarachnoid Hemorrhage. J Emerg Med 2010;39(1):13-16.

Dr Richard Lynch
Consultant in Emergency Medicine,
Midland Regional Hospital,
Mullingar
Ireland

Competing interests: No competing interests

24 March 2015
Richard M Lynch
Consultant in Emergency Medicine
Midland Regional Hospital Mullingar, Ireland
Mullingar, Co. Westmeath, Ireland
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