Diagnostic Accuracy of Chest Radiography for Detecting Residual Pleural Effusion Following Ultrasound-Guided Thoracentesis: An Observational Diagnostic Accuracy Study

1. Introduction

Pleural effusion is the most frequent pleural pathology around the globe due to the causative factors of heart failure, infections, and malignancy. The population and etiology of pleural effusion differ, and about 1.5 million individuals each year are affected in the US alone (Light, 2011). The frequency becomes substantial in hospitalized adults with comorbidities such as heart failure or pulmonary embolism. A chest X-ray (CXR) has high levels of agreement with ultrasound (US) in the assessment of complete resolution of pleural effusion after the use of ultrasound-guided thoracocentesis, but US does a better job in detecting the smaller amounts of residual volume. The US as an equivalent alternative to CXR Direct sensitivity/specificity for CXR in post-thoracocentesis resolution (using the US as a gold standard) remains understudied, but general pleural effusion detection favors the US. Meta-analyses show US pooled sensitivity/specificity of 93%/96% vs. CXR's lower performance (e.g., 42–65% sensitivity, 60–81% specificity). In complicated parapneumonic effusions, CXR sensitivity was 61.5% (95% CI 40.6–79.8%) and specificity 60.0% (95% CI 43.3–75.1%), inferior to US (Karkhanis & Joshi, 2012). Pleural effusion is a common pathology that involves a collection of extra fluid inside the space in between the two layers of pleura, which is the membrane that covers the lungs. The condition may be the result of various underlying diseases and disorders; that is why it is one of the most often realized clinical manifestations in medical practice. Pleural effusion itself is not a primary disease but an outcome of a disease process, which may be benign to life-threatening diseases (Lee, 2016). Consequently, timely identification and proper management of pleural effusion are essential to avoid additional complications and enhance patient outcomes. Pleural effusion may result in severe respiratory compromise and seriously impact the quality of life of a patient when untreated (Porcel et al., 2015). Pleural effusion diagnosis is usually a composite of clinical examination and radiological examination techniques. Imaging has a big role to play in determining the volume of accumulation of fluid, its features, and the possible cause (McGrath & Anderson, 2011). Chest X-ray (CXR) is the most commonly used method of pleural effusion diagnosis as compared to other imaging modalities. CXR is cheap, readily accessible, and simple to conduct, and that is why it has been the first-line diagnostic tool in most healthcare systems. CXR can often rapidly identify fluid in the pleural space, especially if the effusion is large or loculated (Alshmrani et al., 2023). The CXR findings of pleural effusion are commonly based on visual confirmation of pleural blunting of the costophrenic angles or the presence of a meniscus sign, where the fluid will appear as a curved line at the apex of the pleura (Kea et al., 2013). CXR, however, has several limitations, including the detection of small pleural effusions or the accurate quantification of the fluid volume. CXR images are two-dimensional, and therefore, it is hard to identify small and localized effusions, particularly effusions in the posterior or basal parts of the lungs (Gabriella, Kamarga, & Setiawan, 2018). Moreover, CXR may find it difficult to distinguish various pleural effusions, particularly when the effusion is complicated with things like septations, which may indicate malignancy or infection. This weakness is critical in establishing the etiology of the pleural effusion because malignancy and infection need radically different treatment (Rahman et al., 2024).

Ultrasound (US) has achieved growing popularity as a more useful diagnostic option in the diagnosis of pleural effusion, particularly because it allows identifying low volumes of fluid and provides real-time visualization of the pleural space. (Brogi et al., 2017) Ultrasound enables clinicians to visualize the pleural cavity using high-resolution, dynamic images that can be utilized to directly observe the pleural cavity and identify the presence of fluid as well as to direct therapeutic interventions, including thoracocentesis. The real-time health visualization capability to view the pleural fluid means that the ultrasound is capable of not only identifying the presence of the fluid but also of measuring the volume of the fluid, its distribution, and location (Soni et al., 2015). The fact that ultrasound will give comprehensive data on the nature of pleural fluid, including its echogenicity, is one of the greatest benefits of ultrasound (Safai Zadeh et al., 2022). As an example, a complex or septated effusion has an implication of malignancy or infection, whereas an anechoic effusion has a higher probability of being associated with transudative conditions such as heart failure. Other possible complications, including pneumothorax (collapsed lung) or re-accumulation of fluid, are also easily detected by ultrasound and may be hard to see on CXR (Remérand et al., 2010). In addition, ultrasound has also been found to be an indispensable aid in the process of thoracocentesis, which is undertaken to remove pleural fluid (Kalokairinou-Motogna et al., 2010). It also improves the safety and effectiveness of the procedure as the optimal amount of fluid is eliminated, and therefore the risk of fluid reaccumulation in the process is minimized. Real-time control of the process has given way to better results, particularly in those patients with complex or hard-to-find effusions (Evans et al., 2021). In spite of all these benefits of ultrasound, chest X-ray remains the most popular type of imaging in most health care facilities, especially in those with limited resources. The popularity of CXRs is due to their cost-effectiveness and low level of difficulty in use, making them the first option in a wide range of clinical settings (Brockelsby, Ahmed, & Gautam, 2016). Moreover, the reason why CXR is still used in the diagnosis of pleural effusion is due to the familiarity of the clinicians with the interpretation of the CXR and the tradition of using the tool as a first-line diagnostic tool (Lind Plesner et al., 2023). But the inability of CXR to give adequate details of the resolution is one of the greatest weaknesses of the technique in diagnosing pleural effusion of pleural fluid after thoracocentesis. Once the procedure has taken place, the resolution of the effusion will have to be appropriately measured to ascertain the need to either provide further treatment or carry out further procedures like repeat thoracocentesis (Hallifax, Talwar, Wrightson, Edey, & Gleeson, 2017). Unluckily, CXR does not always give this amount of finesse, particularly when the residual pleural fluid is low or when the effusion is situated in areas that cannot be well seen by a typical X-ray film. In other instances, CXR can characteristically miss residual fluid altogether, which can delay clinical decision-making and even unnecessarily repeat procedures (Na, 2014).

On the contrary, ultrasound is reported to provide high diagnostic accuracy in the post-procedural environment. Research has established that ultrasound is more sensitive and specific in the detection of residual pleural fluid and measurement of the quantity of fluid post-thoracentesis. Ultrasound has the capability to give real-time reports on the volume of fluid that still remains in the pleural space, therefore enabling clinicians to make more informed decisions on the subsequent management (Shao, Du, & Xie, 2022). It also makes it possible to evaluate the potential complications in more detail, including the emergence of pneumothorax or the re-accumulation of pleural fluid. Ultrasound is said to be the gold standard in post-thoracentesis assessment due to its potential of giving detailed information and assisting clinical decisions (Marchetti et al., 2015).

This study aims to present evidence of great value by comparing the performance of chest X-ray and ultrasound in the detection of residual pleural fluid and complications after the procedure to enable clinical practice to be informed and better patient care to be offered. This research is important, as it can improve the accuracy of the diagnosis of pleural effusion evaluation, which will eventually result in positive clinical outcomes for patients receiving a thoracocentesis. Specifically, this study will help in the enhancement of clinical practices, imaging planning, and the minimization of unnecessary operations. Additionally, this study will aid in determining the evidence-based decision support and enhance healthcare services in the area with high prevalence rates of pleural effusion, like Pakistan, through the setting up of local diagnostic accuracy statistics.

2. Objective

To assess the diagnostic accuracy of a chest X-ray in showing the resolution of pleural effusion after ultrasound-guided thoracentesis, using the ultrasound as the gold standard, and to find out the similarities in the two imaging modalities.

3. Materials and methods

This observational cross-sectional study was conducted in the department of Radiology and Pulmonology of Hayatabad Medical Complex, a tertiary care hospital in Peshawar, Pakistan, from June to December 2025 for a duration of nine months. The diagnostic test evaluation formula was used to determine the total sample size of 60 patients, and a non-probability convenient sampling technique was used. Patients with age ≥18 years with the clinical diagnosis and imaging-confirmed pleural effusion, who were directed for thoracentesis with ultrasound guidance for diagnostic and therapeutic purposes, were included. Patients who had pre- and post-imaging with chest X-ray and thoracic ultrasound imaging were enrolled in this study. Patients with loculated pleural effusions who were not suitable for complete aspiration, with contraindications to thoracentesis (such as uncorrected coagulopathy), with incomplete imaging data, or lost to follow-up were excluded from study. Prospective data were obtained. Patients were included if they presented with symptoms of dyspnea and chest pain, with initial imaging results.

All the participants undergo chest X-ray and thoracic ultrasound to evaluate the presence, amount, and type of pleural effusion. A structured data collection proforma was used to record imaging findings (site, size, and presence of septations or loculations). Standard aseptic conditions were used, and ultrasound-guided thoracentesis was performed. A post-procedure chest X-ray and thoracic ultrasound were performed to assess the amount of residual pleural effusion and other complications like pneumothorax or recurrence of effusion. A chest X-ray was used to identify the presence of residual effusion and was compared with pre- and post-procedure imaging results to evaluate the efficacy of thoracentesis and the diagnostic value of the chest X-ray. Patients' data were anonymized to maintain patient confidentiality.

All statistical analysis was done with SPSS version 25.0. The continuous variables (age and pleural effusion volume) were presented as mean ± SD, and categorical variables (sex, marital status, occupation, education level, and other clinical parameters) were presented as frequencies and percentages. The chest X-ray was compared to ultrasound as the reference test, and diagnostic performance was assessed through the calculation of sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Agreement of chest X-ray and ultrasound was evaluated by Cohen's kappa statistic. The relationship between the pre- and post-thoracentesis pleural fluid volumes determined by ultrasound was evaluated using paired sample correlation analysis. The p-value <0.05 was regarded as statistically significant.

The study was carried out according to the ethics of the Institutional Review Board. All participants gave written informed consent. All patient information was kept confidential and anonymous with no identifiable information being given. The participants were told that there were no added risks in the study beyond routine clinical care and that they could opt out of the study at any time without repercussions.

4. Results

Sixty patients (50% male and 50% female) with a mean age of 51.47 ± 14.20 years participated in this study. Most of the common presenting symptoms were shortness of breath (30%), fever (23.3%), and cough (20%). The most common underlying conditions were malignancy (25%) and heart failure (23.3%). Initial imaging showed that 98.3% of patients had an X-ray showing pleural effusion, and 100% had an ultrasound examination showing the effusion. Left-sided and bilateral effusions were the most common distributions. The effusions were also characterized using ultrasonography, with anechoic patterns (28.3%) and septated (26.7%) patterns being most common. The volume of pleural effusion decreased on both sides after thoracentesis had been performed, with a significant difference (p < 0.001), suggesting the success of the procedure. On the other hand, residual pleural fluid was seen in 95.0% of the cases on chest x-ray and 98.3% on ultrasound, showing ultrasound was more sensitive to detect small residual collections. On chest X-ray, pneumothorax was found in 58.3% of patients and on ultrasound in 43.3% of patients, and fluid re-accumulation was seen in 71.7% of cases. Septations in residual fluid were found in 51.7% of patients, and further intervention was needed in 41.7% of patients.

The diagnostic accuracy analysis showed that ultrasound was the reference standard, and chest X-ray had a high sensitivity (96.7%), specificity (100%), and overall accuracy (96.7%). The negative predictive value, however, was low (33.3%), meaning limited to ruling out residual effusion. Moderate agreement was found between the two modalities by Cohen's kappa analysis (κ = 0.487; p < 0.001). More importantly, ultrasound was able to detect more cases of residual pleural fluid (RPF) than chest X-ray, which indicated that ultrasound is more sensitive in the assessment of RPF following thoracentesis.

Table 1 summarizes the demographic profile of the 60 participants. The mean age was 51.47 ± 14.20 years, ranging from 26 to 78 years. The sample was equally distributed by gender, with 30 males (50%) and 30 females (50%).

Table 2 presents the presenting symptoms and underlying co-morbidities. The most frequent symptom was shortness of breath (30%) followed by fever (23.3%) and cough (20%). The most common comorbidities were malignancy (25%) and heart failure (23.3%), underlining the high risk profile of the study population.

All cases had pleural effusion detected by ultrasound and distribution similar to a chest x-ray. Table 3 summarizes the echogenicity patterns of anechoic (28.3%), septated (26.7%), and complex effusions (21.7%) and illustrates the fine detail available with ultrasound for characterization and assistance in procedural planning.

Post procedure chest X-ray and ultrasound assessment showed 95% and 98.3% residual fluid respectively (Table 4). Detection rates varied by modality (58.3% for X-ray and 43.3% for ultrasound). This table summarizes the differential sensitivity of imaging techniques for assessment post thoracentesis.

Table 5 evaluates chest X-ray performance using ultrasound as the reference. Sensitivity was 96.7%, specificity 100%, positive predictive value 100%, and negative predictive value 33.3%. Cohen’s kappa indicated moderate agreement (κ = 0.487, p < 0.001). The data support the reliability of chest X-ray for detecting residual effusions, with ultrasound providing confirmatory details.

5. Discussion

The aim of this study was to evaluate the diagnostic performance of chest X-ray (CXR) in the assessment of residual pleural effusion after ultrasound-guided thoracentesis, using ultrasound (US) as the reference standard. In the current study, chest X-ray showed a sensitivity of 96.7%, a specificity of 100%, a positive predictive value (PPV) of 100%, and a negative predictive value (NPV) of 33.3% for residual pleural effusion detection as compared to ultrasound. These results suggest that chest X-ray is good at detecting residual pleural effusion when present, but it has limited ability to confidently rule out residual disease due to its relatively low negative predictive value. In addition, ultrasound could detect more small residual pleural effusion, which proved the better sensitivity of ultrasound for post-thoracentesis evaluation. These findings are compared with those of previous studies, and implications for clinical practice are discussed in the following discussion. Nevertheless, this study is not the only one that demonstrates that CXR is not as sensitive, especially in the identification of small pleural effusion or that CXR can be used to determine residual fluid after thoracentesis. Sensitivity of CXR in this study was determined to be 100, and this is high compared to the sensitivity of 42-65 percent of CXR in the study conducted by MT Kitazono et al., ( 2010) in the diagnosis of pleural effusion (Kitazono, Lau, Parada, Renjen, & Miller Jr, 2010). Nonetheless, the specificity of CXR, in this case, was quite low, 29; thus it is hard to differentiate between pleural effusion and other pathologies that can also cause pleural space blurring in the radiograph, including lung consolidation and atelectasis.

The observation that CXR is low sensitizing when it comes to identifying whether there is a small pleural effusion is supported by a number of studies. According to a meta-analysis by Yousefifard et al. (2024), ultrasound could perform much better with a sensitivity of 94 percent and a specificity of 98 percent in contrast with the 51 percent sensitivity and 91 percent specificity of CXR (Yousaf et al., 2022). This disparity is partly attributed to CXR not being able to distinguish small or loculated effusions that usually go undetected on an ordinary X-ray, particularly when the standing of the pleural effusion is in such locations as the posterior or apical region. This is a limitation that is also aligned with the results of this paper because CXR had trouble detecting the presence of small residual pleural fluid subsequent to the completion of a thoracentesis, hence resulting in the creation of possible mismanagement or redundant repeat surgeries.

In a study conducted by Papadakis et al (2024), ultrasound had 100 percent sensitivity and specificity in identifying pleural effusion and residual fluid following thoracentesis, indicating that ultrasound is better at diagnostic predictability than CXR. In addition, the ultrasound can enable clinicians to dynamically evaluate the pleural cavity and, therefore, understand the arrangement and nature of fluid better. Such real-time capability is vital in the case of a post-thoracentesis where ultrasound may be used to provide intelligible information regarding additional therapy options like the need for repeated thoracentesis (Svigals, Chopra, Ravenel, Nietert, & Huggins, 2017). The results of the study by Andrea Boccatonda et al. (2024) also prove the use of ultrasound in post-thoracentesis evaluations since the authors showed that a patient could have pleural residual fluid despite having a CXR with no abnormalities. This brings out the benefits of ultrasound in postprocedural management and, more so, identifying small fluid collections or other problems such as pneumothorax or fluid reaccumulation that cannot be seen on a chest X-ray (Boccatonda et al., 2024). In a study by MT Kitazono, it is noted that CXR is highly sensitive in the presence of pleural effusion (94%), but not so effective in the presence of fluid leftover following thoracocentesis. This agrees with the Grimberg et al. (2024) study, which identified that CXR was useless in the detection of residual fluid in case of a small effusion or in some hard-to-reach location. In both cases, the failure to identify small or residual effusions in these environments might cause mismanagement or needless repeat operations, which is why the use of ultrasound in the follow-up of thoracentesis patients is of relevance to determine the correct resolution.

Conversely, ultrasound is superior in detecting tiny leftover fluid volumes following thoracentesis; that is the key to determining the need to have additional drainage. The increased real-time imaging through the use of ultrasound enables the clinician to make sound decisions as to whether or not further intervention may be needed and eliminates chances of misdiagnosis. The small size of the NPV of CXR (40% in this study) also highlights why it is difficult to rely on X-ray as a single modality of detection because a false negative might prevent further treatments. The clinical implications of the current study are relevant, especially in the areas that are resource-limited, where CXR is the major form of imaging to detect a pleural effusion. Though CXR remains in widespread use by virtue of its accessibility and cost efficiency, the higher diagnostic accuracy of ultrasound is an issue that should lead healthcare providers to consider the use of ultrasound as an addition to the routine post-thoracentesis care, especially in high-risk patients or in patients with few residual fluid collections.

5.1. Clinical Implications

The results of this study have important implications for routine post-thoracentesis imaging. Chest radiography had high sensitivity in detecting residual pleural effusion, but ultrasound was better in detecting small residual fluid collections and offered real-time assessment of pleural abnormalities. Therefore, routine chest radiography after uncomplicated ultrasound-guided thoracentesis may not be required in all cases if immediate post-procedural ultrasound demonstrates adequate drainage and excludes pneumothorax. In patients with persistent respiratory symptoms, technically difficult procedures, suspected complications, or large malignant or loculated effusions, chest radiography may still provide complementary information. Therefore, ultrasound can be considered the first-line imaging modality in post-thoracentesis assessment, with chest X-ray being reserved for selected clinical situations rather than being a routine examination. The ultrasound-first strategy may reduce unnecessary radiation exposure and healthcare costs and patient waiting time and improve bedside decision-making, especially in hospitals with trained ultrasonographers. In a study by Yousefifard et al. (2024), the results indicated that the implementation of the ultrasound in the management of post-thoracentesis has a positive clinical outcome, especially in finding out the leftover pleural fluid that would otherwise not be seen in CXR. Immediate image displays of fluid accumulations and complications such as pneumothorax provide practitioners with the capacity of making informed decisions on how to take care of the patient and minimize unnecessary operations.

Practical Consequences of Missed Residual Effusions. Small residual pleural effusions missed on chest radiography may not always require immediate intervention. However, in selected patients with persistent symptoms, infection, malignancy, or recurrent pleural disease, failure to detect residual fluid may delay additional drainage or further diagnostic evaluation. Consequently, ultrasound provides greater confidence in determining whether repeat thoracentesis, observation, or additional investigations are required. Also, there is a challenge associated with the operator dependency characteristic of ultrasound, with the inexperienced operators being able to detect the small or loculated effusion. These factors include training the healthcare professionals to utilize ultrasound in order to realize the benefit of ultrasound in the diagnosis. The weaknesses of ultrasound, like a reduced efficacy in highly complex situations, should also be mentioned, and they should be investigated in future developments in order to compare their use with that of other forms of advanced imaging, e.g., CT scans.

Based on our findings, clinicians may consider the following approach:

• Perform ultrasound-guided thoracentesis.
• Immediately reassess using bedside ultrasound.
• If no pneumothorax and only minimal residual fluid are identified, routine chest radiography may be omitted in uncomplicated patients.
• Obtain chest radiography when symptoms worsen, pneumothorax is suspected, drainage was technically difficult, or ultrasound findings are inconclusive.

6. Conclusion

Ultrasound has been shown to have superior diagnostic performance for post-thoracentesis assessment, in part due to its ability to detect small residual pleural effusions and to provide real-time bedside evaluation. Although chest radiography remains readily available and showed high diagnostic accuracy in this cohort, the limited ability of this modality to exclude residual fluid suggests that it should not be routinely relied upon as the sole post-procedural imaging modality. An ultrasound-first approach may result in better patient management, reduced unnecessary radiation exposure, and better utilization of healthcare resources.

7. Limitations

The main limitation on this research is that the sample size was small and may not be a comprehensive representation of the larger population of patients with pleural effusion, thus impacting the generalizability of findings. Also, there is the question of operator dependence of ultrasound, since the accuracy of the results may be affected by differences in technique and experience. The research was also based on a single-center design of the studies carried out at the Hayatabad Medical Complex, which could reduce the ability of the investigation to apply the results to other health care settings, and particularly to places with low accessibility to ultrasound machines. In addition, complications and residual fluid reaccumulation were not always followed in the consecutive long-run period, and this may yield better results in long-term diagnostic performance.

8. Recommendations

According to the findings of this research, it is suggested that ultrasound be used as a standard clinical procedure practice, mainly in the post-thoracentesis ultrasound, because it is highly sensitive and accurate in the presence of residual pleural effusion. Initiatives to improve the competence of healthcare providers in the use of ultrasound, especially in resource-scarce areas, ought to be established through training programs. The incorporation of ultrasound with other high-end imaging modalities, such as CT scans, in the future should include the process of fine-tuning the diagnostic protocols on the management of pleural effusion. Also, bigger multicenter trials that contain follow-up time should be carried out to ascertain these findings and measure the efficacies of ultrasound over the long run in the diagnosis and outcome of pleural effusion.