How well does oblique angle chest X-ray identify occult pneumothorax in trauma patients?

BEEM Bottom Line

Why is this study important?

Occult pneumothorax (PTX) can increase in size and become life-threatening. In patients who cannot undergo computed tomography (CT) scanning, a diagnostic tool besides ultrasound, which is user dependent, would be valuable.

Which, if any, threats to validity are most likely to have an impact on the results and how?

The study excluded unstable patients (cardiac arrest, persistent shock, and those requiring immediate invasive intervention). Since most stable patients with suspected occult PTX will undergo CT scanning, the unstable patients excluded from the study represent those patients to whom the test (oblique X-ray) is most likely to be applied in clinical practice.

How do the key results compare with the current evidence?

The observed sensitivity (62.9%) of emergency physician performed bedside lung ultrasound (LUS) in this study appears lower than that reported in other studies. In a recent meta-analysis, the sensitivity of LUS approaches 89% in detection of all pneumothoraces.[1] One study, specifically looking for occult PTX, found a sensitivity of 92% when performed by emergency physicians with 1 year of ultrasound experience.[2]

How should this study impact the care of ED patients?

Oblique X-ray, similar to LUS, can confirm some cases of PTX not identified on anterior-posterior chest X-ray (APXR). However, the sensitivity is not high enough to rule out the diagnosis in patients in whom there is a moderate or high suspicion of PTX.

Study Summary

Pubmed ID



Matsumoto S, Sekine K, Funabiki T, et al. Diagnostic accuracy of oblique chest radiograph for occult pneumothorax: comparison with ultrasonography. World J Emerg Surg. 2016 Jan 13;11:5.

Study Design

Single-centre, observational, cohort study.


Included: Adult (≥ 18 years) victims of blunt trauma with clinically suspected occult pneumothorax defined as an abnormality (e.g., rib fracture, permeability decay of lung field) on APXR or physical abnormalities (e.g., chest pain, bruise, subcutaneous emphysema) but without overt PTX on APXR.

Excluded: Overt PTX; refractory shock; cardiac arrest or requiring other immediate invasive interventions; transferred from another hospital.

Index Tests

Oblique angle chest X-ray (OXR) consisting of a mobile APXR directed over the pleural interface perpendicular to a film cassette set at 45° against a horizontal plane for each hemithorax.

Reference Tests

All patients underwent CT scan, which was considered the gold standard for diagnosing occult PTX.


Primary: Diagnostic accuracy of OXR.

Secondary: Diagnostic accuracy of LUS; agreement between OXR and LUS.

Key Results

N = 159 (318 thoraces): 70 occult PTX/318 thoraces as defined by the gold standard (CT scan).

Sig. Outcome Result
SS Agreement between OXR and LUS 93.3% (k = 0.804, p < 0.001)
  OXR PPV = 95.6 (95% CI: 0.86 to 0.99)
    NPV = 90.1 (95% CI: 0.87 to 0.91)
    Sensitivity = 61.4 (95% CI: 0.56 to 0.64)
    Specificity = 99.2 (95% CI: 0.98 to 1.00)
  LUS PPV = 93.6 (95% CI: 0.84 to 0.98)
    NPV = 90.4 (95% CI: 0.89 to 0.91)
    Sensitivity = 62.9 (95% CI: 0.57 to 0.66)
    Specificity = 98.8 (95% CI: 0.97 to 1.00)

AUC: area under the (receiver operating characteristic [ROC]) curve; CI = confidence interval; LR = likelihood ratio; Sig. = statistically significant; SS = statistically significant.

BEEM Critique

Risk of Bias Assessment

A1 A2 A3
1 The patients were representative of those likely to undergo testing in the ED. Maybe Maybe Maybe
2 The patients were enrolled consecutively or in a way to ensure a representative sample. Maybe Yes Yes
3 All patients underwent the same diagnostic evaluation. Yes Yes Yes
4 All tests were conducted within similar time frames to preclude changes in disease status. Maybe Maybe Maybe
5 The reference standard criteria for the candidate diagnoses are explicit and reproducible. Maybe Maybe Maybe
6 The reference standard was applied regardless of and blinded to the index test result. No No No
7 The assignment of the candidate diagnoses was explicit and reproducible. Yes Yes Yes
8 Most (> 80%) patients received a diagnosis. Yes Yes Yes
9 Undiagnosed patients received adequate clinical follow-up. Yes Yes Yes
10 The estimates of disease probability are clinically significant. Yes Yes Yes

Funding & Conflicts of Interest



Conflicts of Interest


Potential Threats to Validity


No a priori sample size determination.

Selection Bias

The sampling method is unclear. Since patients with overt PTX were excluded, selection bias may have occurred, whereby less severe patients were included in the study. Also, the number of patients with OPX was small due to narrow entry criteria and may not represent all cases of OPX in the ED.

Measurement Bias

Operator dependence of LUS; PTX can develop and worsen between the OXR, LUS and CT. Since the supervising attending physicians judging the CT scans (gold standard) were not blinded to the results of the OXR or LUS results, diagnostic review bias is possible.

Analysis Bias

There was incorporation bias in the interpretation of both the index and reference tests.






Benton Hunter, MD
Associate Professor of Clinical Emergency Medicine, Indiana University School of Medicine, Indianapolis, IN, United States

Sagar Patel, MD
Senior Resident, Indiana University School of Medicine, Indianapolis, IN, United States


Worster A; Brown J; Hunter B.

Competing Interest Disclosure

Dr. Hunter - No conflicts of interest (ICMJE)

Dr. Patel - No conflicts of interest (ICMJE)


  1. Ding W, Yuehong S, Jianxing Y, et al. Diagnosis of Pneumothorax by Radiography and Ultrasonography. Chest 140.4 (2011): 859 –66.
  2. Soldati G, Americo T, Sara S, et al. Occult Traumatic Pneumothorax: Diagnostic Accuracy of Lung Ultrasonography in the Emergency Department. Chest 133.1 (2008): 204–11.