Diagnostic Accuracy of Focused Assessment with Sonography in Trauma for Hepatic and Splenic Injuries by Blunt Abdominal Trauma using Computed Tomography as Reference Standard

Abdominal trauma is a significant clinical entity leading to emergency hospital admission and is a significant preventable morbidity and mortality. Blunt trauma is more difficult than other types of abdominal injuries, as clinical signs may not be obvious in the early stages, and internal bleeding or solid organ injuries may have occurred. Blunt abdominal trauma can occur from a variety of mechanisms, such as road traffic accidents, falls, occupational injuries and interpersonal violence. The liver and spleen are often affected due to the high vascularity of solid organs and their susceptibility to injury as a result of direct impact, deceleration, or compression (1-3).

In a patient with suspected intra-abdominal trauma, an early diagnosis is crucial. Delayed bleeding from hepatic or splenic injury can lead to shock, longer hospital stays, emergency surgery or death. Earlier than the abdominal wall bruising and altered consciousness, and when there are distracting injuries, clinical examination is often not enough, particularly in patients with early signs of injury but lacking in abdominal wall signs and in the peritoneum. Imaging therefore is a pivotal aspect of evaluating the patient with blunt abdominal trauma (4-6).

In hemodynamically stable trauma patients computed tomography (CT) of the abdomen has become the accepted reference imaging modality. CT can offer detailed anatomic information which can aid in identifying the injured organ, determining the extent and severity of injury, and in decisions for conservative therapy, interventional radiology or surgery. But, there are some restrictions on emergency use of CT. It is more expensive, not all settings may be able to use it at once, will expose patients to ionizing radiation and will require the patient to be moved from the emergency area to the CT suite. These restrictions are especially important in resource-challenged hospitals and those who need a quick bedside assessment (7, 8).

Recent advances in focused Assessment with Sonography in Trauma (FAST) have made it an important first-line imaging modality in emergency trauma care. FAST is rapid, non-invasive, repeatable, portable and bedside-line, and does not delay initial resuscitation. It is mainly used to identify free intraperitoneal fluid, which can be indicative of underlying solid organ injury or internal bleeding. If liver injury is suspected, FAST will reveal free fluid in Morrison's pouch, irregular liver borders or hypoechoic liver parenchyma. It can exhibit perisplenic fluid, splenic contour irregularity, and/or hypoechoic lesions of the spleen in cases of suspected splenic injury. These attributes make FAST useful for initial triage and decision making in a trauma patient (9) (10).

Although FAST has its merits, there are shortcomings. It is operator dependent and will not detect small lacerations, contained hematomas, retroperitoneal injury or early solid organ injury without significant free fluid. It is also affected by obesity, bowel gas, timing of the scan, amount of intra-peritoneal fluid and experience of examiner. Hence, FAST is generally used as a screening tool rather than as a replacement to CT abdomen in stable patients (11, 12).

Some previous studies have shown variable sensitivity and specificity of FAST in blunt abdominal trauma. High accuracy results are reported in some studies and other studies report low sensitivity for certain solid organ injury. These variations might be due to patient selection, severity of the injuries, sonologist experience, timing of the examination, and the nature of the reference standard. Since diagnostic performance can differ between institutions, local operating conditions, resources available and the experience of the operator may vary and diagnostic performance should be assessed in local clinical settings wherever possible (13).

The present study was therefore conducted at the Department of Radiology, Combined Military Hospital, Abbottabad, to determine the diagnostic accuracy of FAST for hepatic and splenic injuries in patients with blunt abdominal trauma, using CT abdomen as the reference standard. The study included 149 patients, as described in the approved synopsis, and assessed sensitivity, specificity, positive predictive value, negative predictive value, and overall diagnostic accuracy of FAST. The findings may help guide clinicians regarding the reliability of FAST as an initial imaging modality and clarify the need for CT confirmation in patients with suspected hepatic or splenic trauma.

There were 149 patients with blunt abdominal trauma in the study. The average age of patients was 34.8 ± 12.6 years and the most common age group was 31–45 years. Males were affected more than females. As described in the synopsis for the purposes of evaluating the diagnostic accuracy of FAST (hepatic and splenic injuries), the reference standard used was CT abdomen.

Table 1. Demographic and Clinical Characteristics of Patients with Blunt Abdominal Trauma

The average duration of the trauma was 7.9 hours ± 4.6. The majority of patients were admitted in the first 6 hours after the injury.

Table 2. Hepatic and Splenic Injuries Detected by FAST and CT Abdomen

The CT abdomen was able to confirm hepatic or splenic injury in 76 (51.0%) patients and the FAST exam was positive in 70 (47.0%) patients. Some patients had involvement of both organs.

Table 3. Cross-tabulation of FAST with CT for Detection of Hepatic or Splenic Injury

64 of 76 CT-confirmed cases were correctly diagnosed by FAST, with an accuracy rate of 83%. There were 12 false negative and 6 false positive cases. There was a statistically significant correlation between FAST and CT findings (χ² = 85.61; p < 0.001).

Table 4. Diagnostic Accuracy of FAST for Hepatic and Splenic Injuries

In blunt abdominal trauma, the FAST was good at detecting hepatic and splenic injuries. The positive predictive value and high specificity suggest that a positive FAST was highly predictive of actual injury on CT.

Table 5. Diagnostic accuracy of FAST According to Organ Involved

Splenic injury was slightly more accurately diagnosed as FAST, than was hepatic injury. Both organ-specific analyses, however, demonstrated statistically significant agreement with the results of the CT.

Table 6. Association of CT-Confirmed Hepatic/Splenic Injury with Demographic and Clinical Variables

Age group and time since trauma were significantly associated with CT-proven liver or spleen injury. There was no statistically significant association between gender and socioeconomic status with CT-confirmed injury.

Among 149 patients, CT abdomen was useful in establishing the diagnosis of hepatic or splenic injury in 76 (51.0%). A total of 70 patients (47.0%) were positive for FAST. The sensitivity of FAST was 84.2%, specificity 91.8%, positive predictive value 91.4%, negative predictive value 84.8% and overall diagnostic accuracy 87.9% when compared with CT. The agreement between FAST and CT was statistically significant (p < 0.001), and FAST was found to be a beneficial initial bedside screening method for hepatic and splenic trauma in blunt abdominal trauma cases

Figure 1. Diagnostic performance of FAST for detecting hepatic and splenic injuries in blunt abdominal trauma using CT as the reference standard

The graph shows that FAST had high specificity (91.8%) and positive predictive value (91.4%), while sensitivity was 84.2% and overall diagnostic accuracy was 87.9%

The present study evaluated the diagnostic accuracy of FAST for detecting hepatic and splenic injuries in patients with blunt abdominal trauma, using CT abdomen as the reference standard. In this study, CT confirmed hepatic or splenic injury in 76 out of 149 patients (51.0%), while FAST was positive in 70 patients (47.0%). FAST showed a sensitivity of 84.2%, specificity of 91.8%, positive predictive value of 91.4%, negative predictive value of 84.8%, and overall diagnostic accuracy of 87.9%. These findings suggest that FAST is a useful initial imaging modality in emergency trauma assessment, especially where rapid bedside evaluation is required. However, the presence of false-negative cases also indicates that a negative FAST scan should not completely exclude solid organ injury when clinical suspicion remains high.

The diagnostic performance presented in this study is similar to the more recent local and international results. The sensitivity, specificity, PPV, NPV, and diagnostic accuracy of FAST for detecting visceral injury in those with blunt abdominal trauma were reported by Siraj et al. (93.94%, 85.37%, 92.81%, 87.50%, and 91.09%, respectively) as a guide to CT (14). Likewise, another study conducted in india showed that the sensitivity, specificity, PPV and NPV of the FAST were 90.3%, 93.6%, 93.4% and 90.5% respectively, with CT as the reference standard for the presence of abdominal visceral injury (15). The slightly lower sensitivity in the present study may be because FAST mostly identifies free intraperitoneal fluid and may not detect early injuries without any significant hemoperitoneum or lacerations to the parenchyma (16).

Of the current study, specificity was higher than sensitivity in the case of FAST. This indicates that a positive FAST response is more likely to indicate an injury than a negative FAST response is to rule out an injury. The results of Ahmad et al. in blunt trauma evaluation were similar, with 78.9% sensitivity, 91.8% specificity, and a total diagnostic accuracy of 84.9%, for FAST (17). A large study by Mohammadi et al. also demonstrated that FAST can be very specific and less sensitive in blunt abdominal trauma, as a screening test rather than a substitute for CT (18). Patent is beneficial in the clinical scenario where it is positive but when negative, further imaging may be warranted in stable patients who have persistent symptoms (19).

In the present study, the splenic injury had slightly greater organ-specific diagnostic accuracy than the hepatic injury. This may be due to the fact that subtle hepatic injuries are more likely to be missed sonographically, whereas perisplenic free fluid and splenic contour abnormalities are easier to appreciate. The sensitivity of organ-specific FAST was 91.4% for liver injury and 84.6% for splenic injury, whereas the specificity of the injury classification was lower for organ-level injury than for intra-abdominal trauma detection in general (20). These differences among the studies indicate that the amount of free fluid, patient body habitus, the type of bowel gas, the quality of the equipment used, the timing of the examination, and the nature of the injury are factors that influence FAST performance.

Clinically important: 12 false negative FAST results in the present study. Some of these can be patients with lacerations in the liver or spleen, or patients with a contained hematoma or patients with injuries with insufficient free fluid to be seen on the initial scan. Intraperitoneal fluid may accumulate over time, and might be better detected by serial FAST. In a 2021 study (21), researchers compared serial FAST at 4 hours, 8 hours, and 12 hours, suggesting that some injuries that may be missed on initial examination can be identified on subsequent exams. In a hemodynamically stable patient, the negative FAST should be read in conjunction with clinical examination, mechanism of trauma, laboratory and CT results (if available).

In stable patients with blunt abdominal trauma, the importance of CT abdomen is still critical due to its ability to provide anatomical detail, assess the extent and grade of injury to solid organs and guide conservative or surgical treatment. Although FAST is fast, non-invasive, repeatable, and helpful in the emergency setting, for stable patients requiring detailed injury assessment, the gold standard is the CT scan. Eastern Association for the Surgery of Trauma guidelines indicate that a follow-up computed tomography scan (CT) can help with management decisions in a hemodynamically stable patient with positive FAST, and prompt surgical intervention may be necessary in unstable patients with positive FASTs. In a similar fashion, clinical reviews state that CT is considered the gold standard for diagnosing intra-abdominal injuries, but that FAST is useful when rapid bedside evaluation is desired.

This study has practical applications in the trauma care of resource-limited environments. FAST may shorten the time to diagnosis and prioritize the patients for CT, surgery consultation, or for more frequent monitoring. The specificity and PPV in this study suggest that a positive FAST should raise the suspicion of an actual hepatic or splenic injury. But since the sensitivity was not 100%, a patient who was negative on the FAST but had strong clinical suspicion should not be dismissed or cleared without further evaluation. This is especially relevant in late presentation after trauma, in patients with ongoing abdominal pain, a falling Hb, tachycardia or high energy mechanisms of injury (22).

There are some limitations of this study. It was performed in one tertiary-care radiology department, which may affect the generalizability of the results to other radiology departments with different trauma volumes, ultrasound machines, or operator experience. The results do not completely apply to critically unstable trauma patients, as they were compared to CT, which is appropriate for stable patients. The scans were also operator dependent and inter-observer variability was not formally assessed, though experienced radiologists performed the scans. Despite these restrictions, the study offers local evidence that is confirmatory of FAST being a fast and reliable screening test for hepatic/splenic injuries in blunt abdominal trauma..

FAST showed good diagnostic performance for detecting hepatic and splenic injuries in patients with blunt abdominal trauma, with 84.2% sensitivity, 91.8% specificity, and 87.9% overall diagnostic accuracy when CT abdomen was used as the reference standard. The high specificity and positive predictive value indicate that a positive FAST scan is strongly suggestive of true solid organ injury. However, false-negative cases were observed; therefore, a negative FAST scan should not be used alone to exclude hepatic or splenic injury in patients with persistent clinical suspicion. FAST should be used as an initial bedside screening tool, while CT abdomen should remain the definitive imaging modality in hemodynamically stable patients requiring detailed assessment.