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Comparison of CT and Sonography in the Diagnosis of Acute Appendicitis: A Blinded Prospective Study

¹ Department of Surgery, St. Elisabeth Hospital Tilburg, Hilvarenbeekseweg 60, 5000 LC Tilburg, The Netherlands.
² Department of Radiology, St. Elisabeth Hospital Tilburg, 5000 LC Tilburg, The Netherlands.
³ Department of Pathology, St. Elisabeth Hospital Tilburg, 5000 LC Tilburg, The Netherlands.
⁴ Department of Medical Statistics, Academic Medical Centre Amsterdam, 1100 DD Amsterdam, The Netherlands.

Received December 3, 2002; accepted after revision May 1, 2003.

 
Address correspondence to P. Poortman (p.poortman{at}elisabeth.nl).

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. Our objective was to compare the accuracy of CT and sonography in a general community teaching hospital for the diagnosis of acute appendicitis in patients with suspected acute appendicitis.

SUBJECTS AND METHODS. In this prospective study, 199 consecutive patients with clinical signs and symptoms of acute appendicitis were examined with sonography (graded compression technique) and CT (focused unenhanced single-detector helical CT [5-mm section thickness]. CT was performed from the L2 vertebral body to the pubic symphysis, and no patients were given oral, rectal, or IV contrast medium. The primary sonographic criterion for diagnosing acute appendicitis was an incompressible appendix with a transverse outer diameter of 6 mm or larger with incompressible periappendicular inflamed fat with or without an appendicolith. The primary CT criterion for diagnosing acute appendicitis was the identification of an appendix with a transverse outer diameter of 6 mm or larger with associated periappendiceal inflammatory changes. The results, independently reported, were correlated with surgical and histopathologic findings.

RESULTS. One hundred thirty-two patients had acute appendicitis at surgery, and 67 patients did not. The sensitivity of CT and sonography was 76% and 79%, respectively; the specificity was 83% and 78%; the accuracy was 78% and 78%; the positive predictive value was 90% and 87%; and the negative predictive value was 64% and 65%.

CONCLUSION. Unenhanced focused single-detector helical CT and graded compression sonography performed in a general community teaching hospital by both body imaging radiologists and general radiology staff members have a similar accuracy for the diagnosis of acute appendicitis.

Introduction

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Traditionally, the diagnosis of acute appendicitis is mainly based on history, findings at physical examination, and results of laboratory tests. The rate of negative findings for appendicitis at laparotomy or laparoscopy based on these parameters may be as high as 50% [1–3]. On the other hand, a delay in the diagnosis and treatment of appendicitis may increase the potential risk of a complicated clinical course [4].

For the past two decades investigators have considered CT and sonography to be accurate imaging techniques for detecting acute appendicitis. Helical CT has reported sensitivities of 70–100% and specificities of 91–99% [5–19]. Sonography has reported sensitivities of 75–90% and specificities of 86–100% [20–23].

Several studies have compared CT and sonography in patients with suspected appendicitis [24–33]. Most of these studies were performed in selected patient groups such as pediatric patients and patients with equivocal clinical signs of acute appendicitis [24, 26, 27, 29–31]. Furthermore, most of the CT and sonography examinations in these studies were performed by body imaging radiologists in university hospitals; only one study was performed in a general community and tertiary care hospital [25]. In a study by Wise et al. [28], radiologists with different degrees of experience evaluated CT scans. This study showed an interobserver variability in the evaluation of the CT scans.

The objective of our study was to assess the accuracy of unenhanced focused single-detector helical CT and graded compression sonography in all patients admitted to a general community and teaching hospital with the clinical diagnosis of acute appendicitis without a selection between typical or atypical clinical signs of acute appendicitis. CT and sonography examinations were performed by both body imaging radiologists and general radiology staff members to reflect a realistic clinical setting.

Subjects and Methods

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Between August 1998 and June 2000, all patients who presented to our emergency department with suspected appendicitis were evaluated by a senior surgical resident or a staff surgeon. If the history, physical examination findings, and laboratory test results raised the suspicion of acute appendicitis, patients were asked to participate in this study. The patients were admitted to the hospital either for observation or for surgery. When admitted between 8 am and 10 pm, patients underwent both CT and sonography. When admitted between 10 pm and 8 am, patients with suspected appendicitis were clinically observed and underwent CT and sonography the next morning, because of logistic considerations in the radiology department. Patients who needed to undergo urgent surgery were excluded. In these cases no imaging was performed. Exclusion criteria were pregnancy and claustrophobia. The radiologic procedures and logistics of the study were explained to the patients, and informed consent was obtained from each patient or from a parent in cases of pediatric patients. The hospitalized patients underwent CT and sonography examinations before undergoing surgery or during the first 24 hr of observation. The decision of whether to operate was based on the clinical parameters and laboratory findings. The operation strategy (i.e., laparoscopy or laparotomy) was determined and documented before CT and sonography were performed.

CT examinations were performed with a single-detector helical CT scanner (Tomoscan AV, Philips Medical Systems, Best, The Netherlands) by means of a rapid thin-scanning technique. A single breath-hold helical scan from the top of the L2 vertebral body to the pubic symphysis was obtained using 5-mm beam collimation and 5-mm/sec table speed (pitch of 1, 120 kV, 100–250 mA). Images were reconstructed and photographed at 3-mm intervals using different soft-tissue window settings (width, 400 H; level, 40 H). In patients younger than 10 years old, the tube current was 100 mA and reconstruction filter 5 was used. In patients between 10 and 15 years old, the tube current was 150 mA and reconstruction filter 5 was used. In patients 15 years or older, the tube current was 250 mA and reconstruction filter 4 was used. No oral, rectal, or IV contrast material was administered. CT scans were analyzed both at a workstation and on hard copy.

In this study, CT findings were interpreted as positive for acute appendicitis when an enlarged appendix ( 6 mm in outer diameter) was identified (Fig. 1). Ancillary signs of appendicitis including right lower quadrant inflammation, appendicoliths, and lymphadenopathy were recorded. CT findings were interpreted as negative if the appendix was visualized with intraluminal air. An appendix less than 6 mm in outer diameter was also diagnosed as normal. If an appendix was not visualized and ancillary signs were or were not present, the findings were interpreted as negative.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Unenhanced CT scan of 26-year-old man shows classical appearance of appendicitis with appendicolith, fat stranding, and 11-mm enlarged appendix (asterisk). Dotted line shows enlarged appendix.

For the sonography examinations, we used 5-12–MHz linear array, 5-2–MHz curved array, and 8-5–MHz curved array transducers (HDI 3000, ATL–Philips Medical Systems, Best, The Netherlands). Curved array transducers were used in obese patients to allow deeper penetration.

Sonography examinations were performed using the graded compression technique described by Puylaert [22]. On sonography, the primary criterion to establish the diagnosis of acute appendicitis was direct visualization of the inflamed appendix: a concentrically layered, small, sausagelike structure found at the point of tenderness. The classic appearance is an incompressible appendix with a diameter of 6 mm or larger and echogenic incompressible periappendicular inflamed fat with or without an appendicolith (Fig. 2). The diagnostic criteria for negative findings on sonography were a compressible right lower quadrant without an enlarged appendix, right lower quadrant inflammation, phlegmon, or abscess. The sonography examinations were performed by resident radiologists under the supervision of radiologists.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —Sonogram of 19-year-old man shows acute appendicitis with incompressible 8-mm appendix (asterisk) and echogenic incompressible periappendicular inflamed fat. Dotted line shows enlarged appendix.

The CT and sonography examinations were performed separately within 1 hr by two radiologists who were unaware of the findings on the other examination. The ratio of the contributions to this study of body imaging radiologists (n = 2) to the other members of the radiology staff (n = 10) was 2:12, which resembles that seen in daily practice. The surgeon was not informed about the radiologic diagnosis. If findings other than appendicitis that had possible clinical consequences were diagnosed on CT or sonography, an independent surgeon was informed. The independent surgeon decided whether the radiologic diagnosis was of consequence for the surgical strategy and whether the operation should be cancelled or the type of operation should be changed (i.e., laparotomy by split-muscle incision or laparoscopy).

The diagnosis of acute appendicitis at surgery was established on the basis of macroscopic findings. A macroscopically normal appendix at laparoscopy was left intact. A normal-looking appendix at laparotomy by a split-muscle incision was excised. All excised appendixes were microscopically analyzed by histology using paraffin sections.

If no diagnosis could be established during the operation and there was still a strong clinical suspicion of an intraabdominal abnormality, the surgeon had two choices: either consult the radiologist immediately or ask an independent colleague surgeon for information about the radiologic diagnosis and consequently determine the operative strategy.

Between August 1998 and June 2000, 339 patients with suspected acute appendicitis were hospitalized. One hundred five patients were excluded. Forty-nine patients were admitted after 10 pm and underwent immediate surgery. Eleven patients underwent immediate surgery during daytime hours because they were judged to be too sick for inclusion in the study. Twelve patients refused to take part in the study. In 14 cases the surgeon forgot or refused to include the patient in the study, and 10 patients were not included because of other logistic problems at the radiology department. Nine patients were not included because there was no clinical suspicion of acute appendicitis, contrary to the general practitioner's view. The remaining 234 patients underwent both CT and sonography.

In eight patients the radiologist considered it necessary to inform an independent surgeon about the radiologic findings before the operation because of possible significant influence on the surgical management of the patient. In four patients the operation was cancelled because both CT and sonography showed diverticulitis. CT scan showed a teratoma of the right ovary in one patient and an epidermoid cyst of the right ovary in another patient. Both patients were operated on by a gynecologist, and the radiologic findings were confirmed at surgery. In one patient CT and sonography showed acute cholecystitis that resulted in laparoscopy followed by conversion and open cholecystectomy. In one patient the independent surgeon decided to continue laparoscopy, although CT and sonography showed inflammation of the terminal ileum, which is suggestive of terminal ileitis. These radiologic findings were confirmed at laparoscopy.

Of the 339 patients considered for inclusion in the study, 226 patients (67%) fully followed the designed protocol. These 226 patients consisted of 125 females and 101 males, ranging in age from 3 to 89 years (mean, 26 years), with six patients younger than 12 years. Most patients (199 [88%]) underwent surgery immediately or within 24 hr of observation after imaging. Twenty-seven patients (12%) were hospitalized for clinical observation after imaging. For this observation group the mean hospital stay was 3 days, ranging from 1 to 7 days. The latter group recovered without surgery during their stay in hospital. The results of CT and sonography in these 27 patients are listed in Table 1. The median follow-up period was 13 months. One patient was readmitted for a delayed appendectomy that microscopically showed a carcinoid. One patient was readmitted 2 weeks later by a gynecologist for puncture of a 4-cm cyst of the right ovary that was seen on both CT and sonography.

All data underwent statistical analysis using the McNemar test. The study protocol was approved by the hospital's ethical committee for human studies.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
At surgery 132 (66%) of the 199 patients proved to have appendicitis, and 67 patients (34%) did not have appendicitis. In 41 (21%) of these 199 patients, another relevant diagnosis was made. In most cases this diagnosis was a gynecologic diagnosis. These diagnoses are listed in Table 2. In the other 26 patients without appendicitis (13%), no diagnosis was made and the appendix was left intact. One of these patients was readmitted 4 months later and proved to have acute appendicitis at laparoscopy.

The sonography results for the patients who underwent surgery are listed in Table 3. In 104 (79%) of the 132 patients with appendicitis at surgery, the sonogram showed signs of acute appendicitis. The other 28 patients (21%) appeared to have acute appendicitis at surgery, although the sonogram showed negative findings for appendicitis.

In 52 (78%) of the 67 patients without signs of appendicitis at surgery, sonography also did not reveal appendicitis. In 15 (22%) of the 67 patients, the sonogram showed positive findings for appendicitis, but a normal appendix was found at surgery. In 20 patients the sonogram was assessed to be suboptimal by the performing radiologist because the patient was obese or was experiencing severe abdominal pain or because the appendix was not visible.

The CT results for the patients who underwent surgery are listed in Table 4. In 100 (76%) of the 132 patients, CT showed acute appendicitis that was confirmed at surgery. The remaining 32 patients (24%) appeared to have acute appendicitis at surgery, although the CT findings were negative.

In 56 (84%) of the 67 patients with a macroscopically normal appendix at surgery, CT findings were negative for appendicitis. In the remaining 11 patients (16%), CT findings were positive.

Statistical Data
The sensitivity of CT and sonography was 76% and 79%, and the specificity was 83% and 78%, respectively. The positive predictive value was 90% and 87%, and the negative predictive value was 64% and 65%. The accuracy of CT and sonography was 78% each. On the basis of the McNemar test results, the calculated p value for sensitivity, specificity, and accuracy was not less than 0.05, which indicates that CT was not superior to sonography in the diagnosis of acute appendicitis.

Histopathologic Findings
During laparoscopy healthy-looking appendixes were not removed. In two of these cases, the patients were readmitted for acute abdominal pain in the right lower quadrant 1 year later. One of these patients underwent laparoscopy, and acute appendicitis was found. Theoretically, a microscopically acute endoappendicitis can be seen by the radiologist and can be erroneously diagnosed by the surgeon as a normal appendix. In three patients microscopic evidence of appendicitis was seen at histology after the surgeon removed a macroscopically normal appendix when performing a split-muscle incision. In one of the three patients CT scans showed appendicitis. In one patient the appendix was microscopically normal, but the surgeon diagnosed an acute appendicitis. In this case, the CT findings also did not suggest appendicitis.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
In this blinded prospective study, CT, as interpreted by the radiology staff in a general community teaching hospital, had sensitivity, specificity, positive predictive value, and negative predictive value similar to those of sonography. As far as we know, this study is the first blinded prospective study comparing the reliability of CT without administration of any form of enteric contrast material (unenhanced CT) and sonography for diagnosing acute appendicitis in daily practice in an unselected population of patients admitted to a general community teaching hospital with suspected acute appendicitis. Wise et al. [28] also performed a comparative assessment of sonography and CT with enteric contrast material to assess acute appendicitis in an unselected group of patients in a university hospital with radiologists with a broad range of experience. Besides showing that CT significantly outperformed sonography, that study showed an interobserver variability in the evaluation of CT scans.

For the continuous effort to reduce the incidence of perforation and negative findings at appendectomy or laparoscopy, CT and sonography are potentially beneficial in the diagnosis of acute appendicitis.

During the past years, graded compression sonography of the right lower quadrant has gained increasing acknowledgment in establishing the diagnosis of acute appendicitis with sensitivities ranging from 77% to 89% and specificities ranging from 94% to 96% [20–23]. The sensitivity and specificity of sonography found in our study confirm these results.

Sonography is relatively inexpensive, rapid, is noninvasive, and requires no patient preparation or contrast material administration. Unfortunately, graded compression sonography is operator-dependent and requires a high level of skill and expertise. Sonography is also a dynamic investigation, and photographs of sonographic images cannot be reliably reevaluated. Another important limitation of sonography is that the sensitivity and specificity for perforated appendicitis are lower than for nonperforated appendicitis [34]. Obese patients and patients with a retrocecal appendix or with severe abdominal pain are difficult to examine using sonography [1]. A radiologist in this study also indicated that these patient-related factors limited the diagnostic capability of sonography.

Other authors reported that CT is an accurate way of imaging acute appendicitis [5–19]. CT is readily available, is supposed to be operator-independent, is relatively easy to perform, and has results that are easy to interpret. Helical CT has reported sensitivities of 90–100%, specificities of 91–99%, accuracies of 94–98%, positive predictive values of 92–98%, and negative predictive values of 95–100% [5–19]. Although oral, rectal, and IV contrast media have been shown to aid in the diagnosis of acute appendicitis, other studies have proven that CT without the administration of contrast material in the setting of suspicion for acute appendicitis can be as accurate as those techniques in which oral, rectal, or IV contrast medium is administered [10, 11, 13, 27, 31]. Therefore, we did not include contrast material administration in our study protocol. Another reason not to administer contrast material was the possibility of patients refusing to enroll in the study to avoid receiving contrast material.

With regard to the accuracy of sonography compared with CT in the diagnosis of acute appendicitis, our sonography results are similar to those of other reports [24–33]. However, the accuracy of CT scans analyzed by the general radiology staff in our study proved to be lower (78%) than the accuracy mentioned in other studies [24–33]. In our study, 11 (16%) of the 67 patients were found to have a normal appendix at surgery, but the CT findings were positive for appendicitis. Possible explanations for false-positive CT results are mild appendicitis, resolved appendicitis, or a reactive enlarged appendix caused by mesenteric lymphadenitis. Thirty-two (24%) of the 132 patients had acute appendicitis at surgery, although the CT findings were negative. These false-negative CT results may be also explained by the fact that less experienced radiologists evaluated the CT scans. In our clinical setting, both body imaging radiologists and general radiology staff members evaluated the CT scans. Wise et al. [28] showed an interobserver variability in the evaluation of CT scans for the diagnosis of acute appendicitis. In that study, the range in accuracy for unenhanced focused appendiceal CT was 75–94%.

Wilson et al. [25] also discussed the fact that in an academic setting the evaluation of CT scans is often done by a CT radiology specialist. In our study, the results were evaluated by radiologists with a broad range of experience. This setup more closely represents the common clinical setting. The disappointing CT results in this study are more likely to reflect the CT performance in an average hospital. Another reason for the lower accuracy of CT in our study may be the exclusion of the possibility for equivocal test results by forcing the radiologist to make a decision whether or not acute appendicitis was present. In daily practice our surgical staff demands a clear statement by the radiologists.

In 41 patients an alternative diagnosis was found at surgery. The cases of diverticulitis, Crohn's disease, cholecystitis, and cecal tumors were detected at both CT and sonography. Of the gynecologic diagnoses, the adnexal teratoma, the epidermoid cyst, and most of the ovarian cysts were seen both on CT and on sonography.

A limitation of this study may be that the radiologists were forced to state whether appendicitis was acute. There was no room for indeterminate answers. We also disregarded the impact of the different degrees of experience among the radiologists in analyzing CT and sonography for acute appendicitis. Disregarded also was the body habitus; in obese patients, sonography may be more difficult to interpret than CT, whereas CT may be more difficult to interpret in thin patients.

In our opinion, the introduction of CT and sonography as a standard procedure in the workup of acute appendicitis can be worthwhile only if the surgeon can rely fully on CT and sonography performed in the hospital. Concern still exists that the overuse or reliance of radiologic tests may distract from careful and timely clinical evaluation and not add significantly to establishing the diagnosis. How high should accuracy of CT and sonography in acute appendicitis be to convince the surgeon not to operate? If a small risk of a perforated acute appendicitis is still present even when both CT and sonography show a normal appendix, most surgeons will neglect the benefits of these additional radiologic tools. Wilson et al. [25] were the first to design a prospective study to determine whether CT and sonography affect a surgeon's decision-making process in acute appendicitis. We believe that further prospective studies are needed to offer a diagnostic pathway in which sonography, CT, and observation can be valuable tools in managing acute appendicitis.

In conclusion, CT for the diagnosis of acute appendicitis in both typical and atypical patients analyzed by the general radiology staff of a general community teaching hospital has an accuracy similar to that of sonography.

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