Bowel obstruction is a clinical diagnosis

You: “Hello Mr/Mrs. Surgical Reg, I’ve got a patient up here on the ward who’s started vomiting tonight and he hasn’t passed flatus for 24 hours and he’s got a tender tummy and I’m worried about a bowel obstruction”

Surgical Reg: “What does his abdo Xray show?”

You: “Its normal”

Surgical Reg: “Well why do you think he has a bowel obstruction then? The Xrays normal!”

Unfortunately when you create work for people, they can sometimes be less than helpful.

Now if you knew the sensitivity of an abdominal Xray, you could answer this question with ease.

This 2007 study (1) showed the sensitivity and specificity to both be about 82%. Second year radiology registrars had sensitivities as low as 59%, while senior radiologists reached up to 93%. Only 29 out of 90 patients had CT proven SBO. CT was the gold standard.

This study (2) used enteroclysis as the gold standard. This involves injecting a contrast material through a NJ tube and taking Xrays. This showed a sensitivity and specificity of 69% and 57%. If you are observant, you might have realised the implication of the numbers going down when a different gold standard is used- CT must not be so great either. Indeed, in this study the sensitivity and specificity of CT were only 64% and 79%.

Part of the explanation for these numbers is that Xray did better at identifying high grade obstructions (86% sensitivity) than low grade obstructions (56% sensitivity). The usefulness of the CT was in showing the cause of the obstruction rather than being far more sensitive.

In contrast (pun intended) this 1999 study (3) showed CT to have a sensitivity and specificity of 93% and 100%, while plain films were 77% sensitive and 50% specific. The gold standard was diagnosis from operative findings (25/30 patients) or by contrast study or clinical follow up, whatever that means, in the remainder.

Lastly this 1997 (4) review article is very informative and worth a read. It describes how the term ‘non-specific gas pattern’ means different things to different people! For example 65% of radiologists use the term to mean ‘probably normal’, 22% mean ‘can’t tell’ and the remainder mean ‘abnormal but not sure if ileus or mechanical obstruction’. What useful terminology!

They identify another analysis (page 1173, paragraph 2) where the sensitivity of plain films was only 66% when read by experienced radiologists; 21% of patients with ’normal’ findings had low grade obstruction.

So we can see that the accuracy of Xrays in the diagnosis of SBO varies a lot depending on the study, but to me seems generally underwhelming. At most the sensitivity in the hands of an experienced radiologist is 93% when compared to CT. It is important to remember however that the usefulness of the sensitivity/specificity depends on the pre-test probability- if your patient is vomiting and not farting and distended, the pre-test probability is high, therefore even with a high sensitivity there will be a large number of false negatives.

Clinical suspicion is important- radiography is a diagnostic aid, not the final arbiter. Always be guided by your clinical findings.

 

References:

  1. William M. Thompson, Ramsey K. Kilani, Benjamin B. Smith, John Thomas, Tracy A. Jaffe, David M. Delong and Erik K. Paulson. Accuracy of Abdominal Radiography in Acute Small-Bowel Obstruction: Does Reviewer Experience Matter? American Journal of Roentgenology. 2007;188: W233-W238. 10.2214/AJR.06.0817
  2. Reliability and role of plain film radiography and CT in the diagnosis of small-bowel obstruction. D D Maglinte, B L Reyes, B H Harmon, F M Kelvin, W W Turner, Jr, J E Hage, A C Ng, G T Chua and S N Gage. American Journal of Roentgenology. 1996;167: 1451-1455. 10.2214/ajr.167.6.8956576
  3. Comparative Evaluation of Plain Films, Ultrasound and CT in the Diagnosis of Intestinal Obstruction. Sudha Suri, S. Gupta, P. J. Sudhakar, N. K. Venkataramu, B. Sood & J. D. Wig. Acta Radiologica. Volume 40, 1999 – Issue 4. Pages 422-428
  4. The role of radiology in the diagnosis of small-bowel obstruction. D D Maglinte, E J Balthazar, F M Kelvin and A J Megibow. American Journal of Roentgenology. 1997;168: 1171-1180. 10.2214/ajr.168.5.9129407

 

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