Usefulness of computed tomography for hospitalized adult patients with fever to investigate cause of fever: single-center, retrospective cohort study

Computed tomography (CT) is a versatile diagnostic imaging tool used in various clinical settings. As Japan has more CT scanners relative to the population size than any other country (111 CT units per 1 million people) [1], clinicians tend to frequently use CT scans to investigate the cause of a fever. Moreover, with the worldwide outbreak of coronavirus disease (COVID)-19 in 2020 and reports of ground-glass opacities on chest CT images even in asymptomatic patients [2, 3], more hospitals have tended to perform CT imaging. On the other hand, the increased use of CT scans may cause harm, such as increased cancer risk from radiation or increased medical costs from overdiagnosis and overtreatment. Indeed, the Nuclear Safety Research Association reported in 2008 that Japanese people were exposed to more radiation, especially from that in medical applications such as CT [4]. Therefore, appropriate use of CT will be more required in the future.

To evaluate the usefulness of CT scans in the investigation of causes of fever, several categories of factors must be considered. First, the urgency determined by a patient’s condition affects the usefulness. Patients with severe hypoxia or hemodynamically unstable status require immediate interventions; therefore, CT would be recommended as part of the initial treatment, because a delay in intervention can affect their prognosis. Second, patient factors such as a present illness or past medical history may affect the usefulness of this imaging modality. For example, aged patients with dementia often have less prominent chief complaints and physical findings, even if they have an infection. In these situations, clinicians perform CT scans more frequently to decide about the treatment. Third, the physician’s skills to assess a patient with fever has an influence. Higher skilled physicians can provide a clearer indication for the CT scan; therefore, a suspicious finding as the cause of fever would be detected more easily on CT images. Fourth, circumstance or cultural background also affects the usefulness. In Japan, the large number of CT units provides such good accessibility to CT scans that physicians tend to use this imaging technique more frequently. Moreover, physicians sometimes defensively perform CT scans to avoid potential litigations. Thus, the usefulness of CT scans for the investigation of the cause of a fever may vary depending on clinical settings.

Patients are categorized mainly into two groups: outpatients, including emergency settings, and inpatients. In the case of outpatients, physicians usually have less clinical information and more time constraints on examining the patients than in the case of inpatients. Especially in emergency situations such as unstable vital signs, the value of CT scans can relatively increase. Conversely, when treating inpatients, physicians should more carefully evaluate the necessity of a CT scan, because they have more information about baseline characteristics and clinical course at hand and have more diagnostic tools available compared to the treatment of outpatients.

Although several studies have reported on the usefulness of CT scans for various infectious diseases, no study has evaluated the usefulness of CT scans in the investigation of the cause of fever [5, 6]. Therefore, as a first step to evaluate the usefulness of CT scans for investigating the cause of a fever, we performed a retrospective cohort study focusing on inpatients. Specifically, we sought to verify the hypothesis that the suspected diagnosis by the treating physician is more significant than blood sampling data to identify on CT a suspicious finding as to the cause of a fever.

To the best of our knowledge, this is the first study to evaluate the usefulness of CT scans to determine the cause of a fever. Two interesting results were found in this study. First, in the case of inpatients with fever, the attending physician’s presumptive diagnosis before the CT scan was associated with the assessment of CT findings. Second, inflammatory responses, as indicated by CRP levels and qSOFA scores, and immunocompromised conditions had limited effects on the CT scan assessment. Furthermore, infectious diseases accounted for most of the causes of fever.

This study showed that the attending physicians should establish a presumptive diagnosis before using CT scans to further investigate the cause of a fever. Many physicians may have already realized that this result is clinically obvious, but our study confirms it. Several publications have reported on the usefulness of CT in certain infectious diseases, whereas there is no study on the usefulness of CT to investigate the cause of a fever. For example, CT has high sensitivity and specificity in the diagnosis of appendicitis. Terasawa et al. [5] conducted a meta-analysis of the diagnostic accuracy of appendiceal CT in adults and adolescents and showed an overall sensitivity of 0.94 (95% CI 0.91–0.95) and specificity of 0.95 (95% CI 0.93–0.96) in 12 prospective CT studies. Doria et al. [6] conducted a meta-analysis and demonstrated that the pooled sensitivity and specificity values for diagnosis of appendicitis in adults were 0.94 (95% CI 0.92–0.95) and 0.94 (95% CI 0.94–0.96), respectively, in 31 prospective and retrospective trials. On the other hand, CT has lower diagnostic accuracy in acute cholecystitis, and one study reported that the correct diagnosis based on CT findings was only 36%, compared to 82% with ultrasonography [8]. Hence, estimating the disease or an underlying cause facilitates selecting the appropriate imaging modality, can reduce the number of unnecessary CT scans, and may increase the usefulness of CT scans.

Second, this study revealed that inflammatory responses, as indicated by CRP levels and qSOFA scores, and immunocompromised conditions had limited effect on the CT scan assessment. The CRP values of the CT-finding group were significantly higher than those of the no-CT-finding group. The difference was a median of 0.5 mg/dL, thus, having little impact on clinical practice. In the multivariate analysis, CRP did not significantly affect the assessment of the CT findings, and similarly, other patient aspects such as qSOFA score and immunocompromised status also did not significantly affect it.

One possible reason for the result that in this study only physician-related aspects were associated with CT findings is the restriction of the study population to inpatients without surgical intervention. During hospitalization, physicians might recognize fever earlier and consequently request a CT scan before inflammatory markers such as qSOFA score and CRP level are increased. Although contrast-enhanced CT provides more information in certain cases such as an abscess or an ischemic lesion, the effect of contrast agent use may have been limited, because radiologists assessed all CT images in this study. It is also possible that the value of the physicians’ initial diagnosis was emphasized in this study, because the patient background was well known, including whether the patient was immunosuppressed or not. Furthermore, the subgroup analysis in immunocompromised patients showed that the CRP value was significantly higher in the CT-finding group. This difference might have been caused by delayed symptom recognition, because these patients tend to have fewer physical findings, including fever, compared to patients with normal immunity. However, the number of cases is small in our study, and further studies are needed to confirm these findings.

CT is also a relatively early assessment method in several structured approaches to investigate fever of unknown origin [9‐11], albeit with different target diseases. However, as Wright and Auwaerter [9] warned, blind application of such algorithms may result in the performance of an excessive number of examinations. Cunha et al. [12] also indicated that imaging modalities should be selected based on fever of unknown origin and the likely pattern of organ involvement. The opinions articulated by these authors are consistent with our results, demonstrating that the physician’s decisions before the CT examination are valuable.

Another study also emphasized the importance of physician-related aspects for the appropriate utilization of imaging modalities [13]. This study was a questionnaire survey to investigate the appropriateness of diagnostic imaging requests, including CT, magnetic resonance imaging, and X-rays, focusing on full-time radiologists at 165 hospitals with an average of approximately 700 beds in Japan. According to the study, the most frequently inappropriate request for an imaging modality was CT (52–61%). Many of the radiologists also indicated that the imaging examinations were inappropriate in the following cases: craniocaudal expansion of the examination range and whole-body imaging with unclear indication. Their study differs from ours in survey methods and topics, but their findings suggest that the aspect of physicians requesting CT examinations is essential for the appropriate utilization of imaging examinations. The results of their study combined with ours suggest that the physician’s decision before the CT examination affects the usefulness and appropriateness of the CT scan.

Our study has some limitations. First, the study was limited to a single center with a small sample size. The eligible patients were also limited to inpatients without surgical interventions. Hence, our result cannot be generalized to other patients with fever, such as after surgery and in emergency settings. Second, we did not know the exact purpose of the CT use due to the retrospective nature of this cohort study. For example, physicians might request a CT scan not only to diagnose or rule out a specific disease but also to perform this scan due to litigation risks. To evaluate the usefulness, we should consider these purposes separately, but we were not able to do so in this study. This may have affected the proportion of detecting a suspicious CT finding regarding the cause of a fever. Third, some of the “suspicious findings” may constitute asymptomatic incidental findings. To sufficiently evaluate this factor, a control group without fever would be required. We should design a prospective study with more detailed physician-related aspects and increased sample size to resolve these limitations.

In conclusion, this study suggests that, in the case of hospitalized patients with fever excluding post-surgical procedures, the attending physician’s estimated diagnosis before the CT scan was associated with the assessment of the CT findings. Estimating the cause of fever before the CT examination, regardless of whether the estimation is consistent with the final diagnosis or not, is expected to increase the usefulness of the CT and lead to more appropriate CT utilization, that is, reducing radiation exposure and the number of unnecessary examinations.