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1 University of British Columbia, Vancouver, BC, Canada.
2 Department of Radiology, Lions Gate Hospital, 231 E 15th St., North Vancouver,
BC V7L 2L7, Canada.
Received June 21, 2004;
accepted after revision September 25, 2004.
Address correspondence to S. G. Bicknell.
Abstract
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MATERIALS AND METHODS. All reports relating to appendicitis were retrospectively obtained from archived transcription reports of nine radiologists from a geographically constrained hospital between December 1999 and December 2003 by a search on the keyword "appendicitis." These files were correlated with the histopathology reports from surgical appendectomy or findings from clinical follow-up during the same period. A survey eliciting the views of five local surgeons on the utility of sonography for the detection of acute appendicitis was also collected.
RESULTS. Sonography reports for 667 patients (mean age, 34 years; range, 6-93 years) were obtained. Of these, a total of 174 had pathologically proven appendicitis and 145 had positive findings for appendicitis on sonography. The accuracy was 92%; sensitivity, 83%; and specificity, 95%. The positive predictive value was 86%, and the negative predictive value was 94%. Three of the five surveyed surgeons indicated they used sonography less than 25% of the time, with none using it more than 75%.
CONCLUSION. The sensitivity, specificity, accuracy, and positive and negative predicative values of sonography performed by general radiologists in a community hospital are comparable to statistics quoted in the literature for academic institutions. The most common error was the tendency to misclassify appendixes under 6 mm. Most surgeons surveyed stated their use of sonography would increase if sonography yielded a sensitivity and specificity of 85% or greater.
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To reduce the rate of negative appendectomy and to improve the sensitivity of the diagnosis, physicians use sonography, one of many techniques used. The method of graded compression sonography is well established by several large prospective trials that have reported sensitivities of 77-89% and specificities of 94-96% [4-8]. In all of these prospective studies, evaluations were performed directly by radiologists who often had subspecialty training in sonography; thus, these results may only be a guide in estimating the actual values in an average community hospital with a general radiologist performing and interpreting the examination. The primary aim in this study was to evaluate the diagnostic accuracy, sensitivity, and specificity of sonography in the evaluation of acute appendicitis in a community hospital setting. The utilization of and attitude toward sonography by general surgeons were also assessed.
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Sonography Technique and Interpretation
All abdominal sonograms were obtained by nine general radiologists with
experience ranging from 2 to 25 years. Examinations were performed on an ATL
3000 or 5000 unit (Philips Medical Systems) using both curved array 3-5-MHz
and linear array 7-10-MHz transducers. All radiologists used the graded
compression technique previously described by Puylaert
[7]. Three criteria were
generally used for the diagnosis of appendicitis: enlargement, lack of
compressibility, and having a blind ending. An appendix was considered
enlarged when the maximal cross-sectional diameter under compression was
greater than 6 mm.
Follow-Up Procedures
For patients who underwent appendectomy, the sonography findings were
compared with the microscopy report as the gold standard. Of the patients with
false-positive findings on sonography, each specific sonography report was
obtained and analyzed for the presence of the diagnostic criteria described
earlier if the patients went to surgery. If the patient did not undergo
surgery based on the surgeon's opinion, the family doctor was called for
follow-up. Our hospital is relatively isolated, so any negative examination
without surgery was interpreted as a true-negative because patients are
unlikely to drive to another hospital for another opinion. The outcomes of all
667 patients' reports were assessed. In addition, five local general surgeons
were asked to complete a questionnaire
(Appendix 1) at the conclusion
of the study detailing their use of sonography and their perceptions of its
sensitivity and specificity.
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Statistical Analysis
The sensitivity, specificity, accuracy, positive predictive value, and
negative predictive value of sonography in the detection of appendicitis were
calculated. Subsets of sensitivity, specificity, and positive predictive value
were also analyzed using age as a discriminator (
10, 11-18, 
19
years).
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Of the 667 patients identified in the radiology database, 174 had acute appendicitis and 493 did not. According to the pathology database, the total number of appendectomies performed during the study analysis was 788. One hundred forty-five of those 174 patients with appendicitis had positive findings on sonography, and 29 had negative findings. Thirty-one of the 145 patients with true-positive findings had a ruptured appendix at the time of surgery. Of the 29 patients with false-negative findings, four had suboptimal examinations due to obesity. The sonography reports for all four patients recommended follow-up CT, which subsequently revealed acute appendicitis in two patients and a ruptured appendix in the other two patients.
All of the 174 patients with acute appendicitis had the diagnosis confirmed by surgery and the subsequent histopathology report. All 23 patients with false-positive sonography findings underwent surgery. Three of the patients from the false-positive group needed surgery anyway: one had ruptured cecal cancer, another had an appendiceal mucocele, and the final patient had diverticulitis with abscess formation. Two other patients had pelvic inflammatory disease and an omental infarct. The 470 patients with true-negative sonography findings were followed up by calling their family doctor. Table 3 shows the sonographic appearance of all the false-positive cases. All radiologists at the hospital made one or more false-positive diagnoses, with the most common problem being that they included some patients whose appendix measured less than 6 mm.
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Three surgeons specified that they used sonography less than 25% of the time, one from 25% to 50% of the time, and one from 50% to 75% of the time. All five surgeons stated that their use of sonography has increased over the past 4 years. Table 4 displays the factors that most positively influence them to refer a patient for sonography of the right lower abdomen. The top three factors ranked were if it were a pediatric case, a case with equivocal findings, or no increase in WBC was noted. Obesity was mentioned as a factor three times. Four surgeons believed the sensitivity and specificity of sonography for the diagnosis of appendicitis to be between 70% and 90%, with the other indicating 50-70%. Three surgeons indicated that they would increase their use of sonography if the sensitivity and specificity both were more than 85%, whereas the remaining two surgeons would not.
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Sonography has the inherent advantages of being relatively inexpensive, rapid, non-invasive, radiation-free, and dynamic, and no patient preparation is needed. Its drawback is that it is highly operator-dependent, thus requiring a high level of expertise and skill. The static recorded images are not reliably interpreted later. The method of graded compression sonography of the right lower quadrant has become widespread in the diagnosis of acute appendicitis because it is reproducible.
A large set of appendiceal and periappendiceal criteria are used to diagnose acute appendicitis, with the most sensitive and specific being a diameter of 6 mm or greater (sensitivity, 98%; specificity, 98%), lack of compressibility (sensitivity, 96%; specificity, 98%), and inflammatory fat changes (sensitivity, 91%; specificity, 76%) [8]. In fact, a visualized appendix measuring less than 6 mm in diameter has a reported negative predictive value of 98-100% [8-10]. Indeed, seven of the 23 false-positives in our study had an appendix diameter of 5 mm, making it the most common error in cases of misdiagnosis. However, to revise the diameter criterion to a higher threshold is to lower sensitivity in favor of specificity; because of the severity of a missed diagnosis, surgeons have traditionally preferred a more sensitive test.
A nonvisualized appendix also presents a major diagnostic difficulty because one cannot confidently exclude appendicitis without examining the appendix. Conflicting rates of visualization have been reported for the visualization of normal appendixes—from 0-4% [11] to 64-72% [8, 10]. Not surprisingly, Kessler et al. [8] and others have also reported that nonvisualization of the appendix has a negative predictive value of 90%, but only for sonographers who routinely find normal appendixes [8]. On the other hand, Himeno et al. [12] reported a rate of 18.3% of appendicitis in nonvisualized appendixes with an overall rate of visualization of 70.6%. We routinely do not identify or comment on normal appendixes because we believe this to be unreliable.
An important limitation of sonography in the examination of patients with perforated appendix is its lower sensitivity compared with nonperforated cases [13]. In addition, some authors have claimed the additional time of any imaging technique (most commonly CT and sonography) leads to longer times before definitive treatment and a higher risk of perforation. In their recent study, van Breda Vriesman et al. [14] assessed the length of hospital delay and the appendix perforation rate in patients examined with sonography and optional CT: the perforation rate (21.9%) was comparable to those reported in the nonimaged patients in surgical studies (range, 16-28%; median, 20-22%). In addition, the median length of hospital delay for our study population (5 hr) compares favorably with reported delays (6-9 hr) [14]. In our study, of the 174 patients with appendicitis, the appendix had ruptured in 31 cases (18%), which lies well within the range of the value reported for nonimaged patients in the literature. One possible reason why the number of perforated cases was lower in our study population is that the patients with such cases are usually taken to the operating room without undergoing sonography.
Most of our local surgeons report using sonography less than 25% of the time, although they state their use of sonography for the diagnosis of appendicitis has increased in the past 4 years. Views about the utility of sonography examination in the diagnosis of appendicitis vary. In some institutions, sonography is considered routine [14], whereas other groups claim that clinical examination by an experienced surgeon has an accuracy of 71-97% and recommend sonography in only select cases [15-18]. The three top factors selected from the five factors that influence them to ask for sonography by our local surgeons were pediatric cases, cases with equivocal findings, and no increased WBC.
The reported range of sonographic accuracy in determining acute appendicitis in children has been examined by numerous authors [19, 20]. Orr et al. [17] reported from their meta-analysis of all the pediatric studies performed between 1986 and 1994 an overall sensitivity of 85% and specificity of 92% for sonography in the diagnosis of appendicitis in children. Sivit et al. [21] in a more recent study reported a sensitivity of 78%, a specificity of 93%, and an accuracy of 89%.
Although the number of children in our series is small, we report
sensitivities and specificities (age 10 years, sensitivity of 82% and
specificity of 94%; age = 11-18 years, 86% and 94%, respectively) similar to
those reported by other authors. In terms of equivocal findings and
nonelevated WBC, sonography may be a useful adjunct to clinical and laboratory
evaluations, which is also the view of the surgeons surveyed. Although obesity
is a well-recognized factor that severely limits the performance and
interpretation of any sonographic examination, it may also hinder the physical
examination, leading to diagnostic uncertainty. This factor would probably
influence the surgeon to request sonography, even if sonography is known to be
of low prognostic value. CT would be the preferred imaging method in this
case; however, at our institution, CT for appendicitis is used only as a
problem-solving tool because we have only one CT scanner.
The limitations of this study include those of any retrospective study. However, the selection bias in this case favors a lower sensitivity, specificity, and accompanying predictive values because the cases for which sonography is performed will more likely be difficult with equivocal findings. In addition, cases of self-limiting and spontaneously resolving appendicitis have been reported in the literature [22, 23]. Also, patients could have gone to the main center hospitals after the assessment at our hospital, although this is unlikely. Both these factors would tend to underestimate the false-negatives and overestimate the false-positives. However, it is difficult, if not impossible, to resolve these obstacles. Finally, the number of general surgeons at our hospital is limited, and we cannot assume their views necessarily reflect the norm. It would also have been interesting to compare the accuracy and utility of CT in this general community hospital setting, but as mentioned earlier, CT is uncommonly used in our hospital for the evaluation of appendicitis.
In conclusion, the sensitivity, specificity, accuracy, and positive and negative predictive values of sonography performed by general radiologists in a community hospital are comparable to statistics quoted in the literature for academic institutions. The most common error was the tendency to misclassify appendixes less than 6 mm as appendicitis.
Acknowledgments
We thank Ivan Chan in assisting with proofreading the manuscript and
Michele Stasiuk in assisting with the database search.
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