Article of the Week: Effect of Duration of Abdominal Pain on Accuracy of Imaging in Appendicitis

by Jamil Madati
Children's National

For this week’s article I decided to go back a couple of months to an article that came out in the Annals of Emergency Medicine November last year that put data to what I (and probably most of you) suspected clinically. We evaluate patients for possible appendicitis all the time, and often these patients present to the ED with several hours of symptoms. This might be obvious to some but I always suspected that the shorter the duration of symptoms (in this case duration of abdominal pain) the less likely that CT or US would be able to pick up appendicitis. Basically, this study put hard data to back up that assumption by looking at the test characteristics (sensitivity and specificity) for CT or US with relation to the duration of abdominal pain symptoms.

To quickly summarize the methodology:

Study Design: Secondary analysis of data from a multicenter prospective observational study which collected data to try to refine an appy prediction rule. (9 different sites)

Inclusion criteria: Age 3yo to 18yo, presenting with acute abd pain (duration less than 72hrs) whom the clinician was suspicious for “possible appendicitis”. (“possible appy” clearly defined on p583)

Exclusion criteria: pregnancy, previous abd surgery, chronic GI conditions, severe developmental delay, prior radiographic studies from outside institution.

Outcome measures: Presence or absence of appendicitis (as defined by pathology report on operative specimen). Clear definitions of what was considered a “normal” vs “positive” vs “equivocal” radiological study for both CT and US were clearly defined and outlined a priori and can be read on p584.

N=1810. Duration of abdominal pain symptoms split into 5 categories: less than 12hrs, 13-24hrs, 25-36hrs, 37-48hrs, 49-72hrs

Results and discussion:
Majority of patients in this study population underwent CT as part of their evaluation n=1216, whereas n=832 underwent US (which when compared to our population at CNMC, I suspect we do a lot less CTs). Also a majority of the patients enrolled presented within 24hrs of the onset of abdominal pain n=1120 out of 1810 (with 53% of these folks presenting between 13-24hrs of onset of pain, which, in my brief experience, is generally what we see here). The researchers calculated the test characteristics (sensitivity, specificity, PPV and NPV) for each time interval for both CT and US. They looked at test characteristics for “positive” CT and US reads (excluding “equivocal” reads) and then again for when the CT or US were read as “positive” or “equivocal” (See table 2 on p587).

Major Finding of this study:
US sensitivity increased with linearly with the increased duration of abdominal pain symptoms 81% sensitivity at less than 12hrs, increasing the 96% at greater than 48hrs (i.e. the longer the pain, the more sensitive US was for picking up appy). However, with CT, sensitivity stayed pretty much (greater than 94%) the same regardless of duration of abd pain symptoms (for those who are visually inclined, see the beautiful graphs on Table 2 p587)

For the significant number of those patients we see that present to the ED with abd pain less than 12hrs, for whom we are suspicious for acute appy, the sensitivity of US is good (~81%) but not great. This is not to say that you should not order the test to rule it out if you are clinically concerned but just know that an US early in the course of symptoms isn’t as accurate as it would be later in the course of symptoms. I would strict return precautions, as well as close follow up with PCP within 12-24hrs for repeat abd exam. This is where things like appy scoring systems or Dr. Atabaki’s biomarker for appy might be of help for us when deciding what to do with those patients with equivocal histories or physicals and negative or equivocal US reads. However, CT has a pretty high sensitivity (>94%) regardless of the time course of symptoms (although I would suspect that it too has a poor sensitivity if done within 4-6hrs of onset of abdominal pain). But with the push to limit radiation, and the fact that CT appy protocol at CNMC requires both oral and IV contrast this would not be best option for our patients presenting early with symptoms.

The study has other interesting talking points (study design, follow up on missed patient, decrease in CT equivocal readings with increase in time, NPV of the above studies etc) but for the sake of brevity I’ll stop here.

Enjoy the read (and the lovely spring weather… finally! No one warned me about the wicked allergies!)

Bachur, RG et al, The Effect of Abdominal Pain Duration on the Accuracy of Diagnostic Imaging for Pediatric Appendicitis. Ann Emerg Med. 2012;60(5):582-90