The hallmark finding of a pneumothorax on a chest X-ray is a thin white line running parallel to the chest wall, representing the edge of the lung (the visceral pleura), with no lung markings visible beyond it. Spotting that line sounds straightforward, but in practice it can be surprisingly difficult, especially when the patient is lying flat or when skin folds, tubing, or underlying lung disease create confusing shadows. The steps involved go well beyond “look for the line,” and the imaging approach you use and the position the patient is in can dramatically change whether the pneumothorax shows up at all.
The Classic Sign on an Upright Film
On a standard upright posteroanterior (PA) chest X-ray, free air in the pleural space rises to the apex of the lung. You look for a fine, sharply defined white line at the lung’s edge that curves parallel to the inner chest wall. On the far side of that line, toward the ribs, you see no blood vessels, no bronchial markings, just a darker-than-expected space. On a lateral view the same pleural line can appear in the space behind the breastbone or overlying the vertebral bodies, again running parallel to the chest wall.1BMJ. Radiological review of pneumothorax That combination of a visible pleural edge plus absent distal lung markings is what separates a true pneumothorax from the various look-alikes that trip people up.
Accuracy improves when the patient is upright. In one study comparing upright and decubitus (side-lying) films read by five radiologists, pneumothoraces were detected about 21% more often on the upright images, and the readers were more confident in their calls.2PubMed. Pneumothorax: detection with upright versus decubitus radiography When it is safe to sit a patient up, an upright film is the best starting point.
Why Supine Films Make Things Harder
Many patients who need urgent imaging, particularly trauma patients immobilized on a backboard, can only have their X-ray taken while lying flat. In the supine position, free air does not collect at the apex. Instead it pools anteriorly, settling in front of the lung where it has no clear edge visible on a front-to-back projection. Supine chest X-rays are considerably less sensitive for pneumothorax than upright films, and the features surgeons and emergency physicians need to hunt for are different from the classic apical finding.3PubMed Central. The supine pneumothorax
The most well-known supine clue is the “deep sulcus sign.” Normally the angle where the diaphragm meets the lateral chest wall (the costophrenic angle) has a gentle curve. When air tracks down to the base of the pleural space in a supine patient, that angle becomes abnormally deep and sharp on the affected side. In some cases, the deep sulcus sign is the only radiographic evidence of a pneumothorax.4PubMed. The deep sulcus sign A related finding is the “double diaphragm sign,” where air outlines both the top and the base of the diaphragm, creating two visible lung-diaphragm interfaces on the same side.5Annals of Emergency Medicine. A Supine Oblique Chest Radiograph Method for Detecting Occult Pneumothorax
Other subtle supine findings include increased sharpness of the heart border or mediastinal edge on the affected side (because air now outlines those structures), and a generally more lucent (darker) hemithorax that is easy to miss unless you deliberately compare the brightness of both lung fields side by side.
The Occult Pneumothorax Problem
A pneumothorax that is invisible on a supine chest X-ray but turns up on CT is called “occult.” The term is not academic trivia: occult pneumothoraces account for a strikingly large share of all traumatic pneumothoraces, with estimates ranging from roughly half to nearly two-thirds.5Annals of Emergency Medicine. A Supine Oblique Chest Radiograph Method for Detecting Occult Pneumothorax The supine anteroposterior (AP) chest X-ray is the most practical initial study in trauma patients who need cervical spine immobilization, but it simply is not as sensitive as CT for early detection.6PubMed Central. Occult pneumothorax, revisited.
Clinicians sometimes add a supine oblique view, tilting the patient slightly toward the suspected side, to bring an anteriorly located air collection into a position where its pleural edge becomes visible. This is a low-tech workaround when CT is not immediately available but clinical suspicion is high. Any time a supine film looks negative yet the patient has chest-wall tenderness, subcutaneous emphysema, or a mechanism of injury consistent with a pneumothorax, further imaging with CT or ultrasound is warranted.
Does an Expiratory Film Help?
You may have heard the teaching pearl that an X-ray taken at end-expiration makes a small pneumothorax easier to see, because the deflated lung pulls away from the chest wall and the air gap looks bigger. The evidence tells a more nuanced story. A controlled comparison of upright inspiratory versus expiratory films found no meaningful difference in detection: the average area under the curve was essentially identical for inspiration and expiration, and neither sensitivity nor specificity changed.7PubMed. Comparison of upright inspiratory and expiratory chest radiographs for detecting pneumothoraces
Where expiratory films do make a difference is in estimating size. One study found that the measured pneumothorax size was on average about 9% larger on expiratory films compared with inspiratory ones, and that difference was enough to change the recommended treatment category for roughly 14% of patients.8PubMed. What is the difference in size of spontaneous pneumothorax between inspiratory and expiratory x-rays? So while expiration does not help you detect a pneumothorax you would otherwise miss, it can push a borderline case across a treatment threshold, which matters when guidelines use a size cutoff to decide between observation and intervention.
Estimating Size on the Film
Once a pneumothorax is identified, the next question is how big it is, because size often guides whether to watch and wait or insert a drain. Several formulas exist, and none are perfect. The most widely cited approach measures the distance between the lung edge and the chest wall at specific points, then plugs those measurements into a regression equation. One study validated a formula using three interpleural distance measurements correlated against helical CT volumes, finding a strong relationship that could predict the percentage of lung collapse with good accuracy.9PubMed. Quantification of pneumothorax size on chest radiographs using interpleural distances: regression analysis based on volume measurements from helical CT
In practice, different formulas give slightly different numbers. A comparison of two commonly used methods (Collins and another standard approach) found that they agreed closely for small pneumothoraces but diverged as size increased, with the Collins method tending to estimate a larger percentage. The average difference was about 4%, but the range of disagreement was wide enough that the two methods could lead to different clinical decisions in borderline cases.10PubMed. Comparison between two methods for estimating pneumothorax size from chest X-rays Reader experience and even the type of display monitor being used can also affect measurements, adding another layer of variability.11PubMed Central. Evaluation of three pneumothorax size quantification methods on digitized chest X-ray films using medical-grade grayscale and consumer-grade color displays
The takeaway is that X-ray-based size estimates are useful approximations, not precise measurements. When the clinical decision hinges on whether a pneumothorax crosses a guideline’s size threshold, clinicians should be aware that the number can shift depending on the method, the film, and who is reading it.
Skin Folds and Other Look-Alikes
The single most common mimic of a pneumothorax on a chest X-ray is a skin fold. When the X-ray cassette presses against a patient’s back, it can bunch up a fold of skin that casts a line across the image. In trauma patients, where clothing is cut away and patients are moved quickly, skin folds are especially common. Misreading a skin fold as a pneumothorax can lead to an unnecessary chest tube, which is not a trivial intervention.
There are reliable ways to tell them apart. A true pneumothorax produces a thin, sharply defined line (the visceral pleura) with increased darkness beyond it and no lung markings crossing it. A skin fold creates a broader, more diffuse opacity, and critically, you can usually trace blood vessels and lung markings right through and beyond the line. The outer edge of a skin fold is often sharply outlined by a bright Mach band artifact, which can actually make it look more convincing at first glance. The path of the line matters too: a skin fold does not follow the expected anatomical curve of the pleura, and it may extend beyond the lung field entirely, crossing the mediastinum or running off the edge of the image.12Annals of the American Thoracic Society. Differentiating Pneumothorax from the Common Radiographic Skinfold Artifact
A case report from a Korean trauma center illustrates the real-world stakes: a skin fold on a chest X-ray was initially misread as a pneumothorax, prompting unnecessary intervention. The authors emphasize that location, shape, sharpness of the line, and the presence or absence of vascular markings on the far side are the key distinguishing factors.13PubMed Central. Misinterpretation of a skin fold artifact as pneumothorax on the chest x-ray of a trauma patient in Korea: a case report
When Underlying Lung Disease Complicates the Picture
Patients with pre-existing lung disease can make pneumothorax detection genuinely difficult. In emphysema, giant air-filled bullae (large thin-walled spaces within the lung) can mimic a pneumothorax on plain film, because both appear as lucent areas with thin walls. One described way to distinguish them is the “double-wall sign”: if air from a pneumothorax surrounds a bulla, you can see both the inner and outer surfaces of the bulla wall outlined by air, much like the classic double-wall sign used to detect free air in the abdomen.14PubMed. Usefulness of the double-wall sign in detecting pneumothorax in patients with giant bullous emphysema
Pulmonary fibrosis presents a different challenge. In patients with idiopathic pulmonary fibrosis (IPF) who develop sudden breathlessness, chest X-rays missed extra-alveolar air in half of the cases examined in one series. CT was needed to find the pneumothorax or pneumomediastinum that the plain film failed to reveal.15PubMed. Spontaneous pneumothorax and pneumomediastinum in IPF The dense, irregular parenchymal markings of fibrosis make it hard to identify the pleural line, and a small pneumothorax can hide behind a background that is already abnormal. For these patients, a low threshold for CT is reasonable when symptoms suggest a pneumothorax and the plain film is ambiguous.
Ultrasound as a Faster Alternative
Bedside lung ultrasound has become a serious competitor to the plain chest X-ray for pneumothorax detection, particularly in emergency and trauma settings. A systematic review and meta-analysis of studies performed by emergency healthcare workers found that chest ultrasound had a sensitivity of about 79% compared with about 48% for chest X-ray, while both had nearly identical specificity near 99.5%.16PubMed Central. Chest ultrasound vs. Radiograph for pneumothorax diagnosis performed by emergency healthcare workers in the emergency department: a systematic review and meta-analysis A Cochrane review focused specifically on trauma patients found an even starker gap, with ultrasound sensitivity around 91% versus 47% for supine chest X-ray, and no meaningful difference in specificity.17Cochrane Database of Systematic Reviews. Chest ultrasonography versus supine chest radiography for diagnosis of pneumothorax in trauma patients
Ultrasound detects a pneumothorax by looking for the sliding motion of the two pleural layers against each other during breathing. When air separates them, that sliding disappears. An individual prospective study at a trauma center found ultrasound correctly identified pneumothorax in about 90% of confirmed cases, compared with roughly 69% for supine X-ray.18PubMed Central. Diagnosis of Traumatic Pneumothorax: A Comparison between Lung Ultrasound and Supine Chest Radiographs The advantage is most pronounced in exactly the situation where X-rays struggle most: the supine trauma patient. Ultrasound is also portable, radiation-free, and can be done at the bedside in seconds, which explains its growing role in emergency protocols. It does have limitations, including difficulty in obese patients, those with extensive subcutaneous emphysema, and situations where bullous disease or prior pleurodesis alter normal pleural sliding. Still, in trauma resuscitation, ultrasound has effectively replaced the supine X-ray as the first-line screen at many centers.
Pneumothorax After Procedures
Not all pneumothoraces are spontaneous. CT-guided needle biopsies of lung lesions cause a pneumothorax in roughly one in five patients, though reported rates range from about 9% to over 50% depending on the study and patient population.19PubMed Central. Pneumothorax after transthoracic needle biopsy of lung lesions under CT guidance The factors most consistently linked to higher risk include smaller lesion size, greater depth of the lesion from the pleural surface, and the presence of emphysema. Interestingly, one study found that male sex was an independent predictor of post-biopsy pneumothorax, though most series have not confirmed this.20Danish Medical Journal. Iatrogenic pneumothorax after CT-guided lung biopsy
Standard practice after a lung biopsy is to obtain a follow-up chest X-ray, usually around two hours post-procedure, to check for a delayed pneumothorax. Some centers also do an immediate CT scan right on the table. One study found that an immediate post-biopsy CT detected about 80% of the pneumothoraces that were later confirmed on the two-hour X-ray, but it had a negative predictive value of only 94%, meaning about 6% of patients with a clean immediate CT still developed a visible pneumothorax by the time the later X-ray was taken.21PubMed Central. The yield of immediate post lung biopsy CT in predicting iatrogenic pneumothorax The two-hour X-ray remains important even when the immediate scan looks clear.
AI-Assisted Detection
Deep learning algorithms trained on large datasets of chest X-rays can now flag suspected pneumothoraces with impressive accuracy. One deep learning system tested across six external institutions achieved an area under the curve between 0.91 and 0.98 for pneumothorax detection, holding up well outside the hospital system where it was trained. Performance was lower for small pneumothoraces than for large ones, which mirrors the same pattern seen with human readers.22PubMed Central. Deep Learning Systems for Pneumothorax Detection on Chest Radiographs: A Multicenter External Validation Study Another model focused on moderate and large pneumothoraces reached a sensitivity of about 84% and specificity of 90% in its high-sensitivity configuration.23PLOS Medicine. Automated detection of moderate and large pneumothorax on frontal chest X-rays using deep convolutional neural networks: A retrospective study
The more interesting finding is what happens when AI assists human readers rather than replacing them. In a study testing readers of varying experience levels, AI assistance significantly improved performance across all groups, with the biggest gains among junior frontline providers such as emergency physicians and nurses. The AI essentially narrowed the gap between experienced radiologists and less-experienced clinicians.24PubMed Central. Impact of AI Assistance in Pneumothorax Detection on Chest Radiographs Among Readers of Varying Experience These systems are increasingly deployed as triage tools that bump suspicious films to the front of a radiologist’s reading queue, reducing the time between image acquisition and someone noticing a potentially dangerous finding.
Neonates and the Spinnaker-Sail Sign
Newborns have their own set of radiographic quirks. The thymus is large relative to the chest, and free air in the mediastinum (pneumomediastinum, which can accompany a pneumothorax) lifts both lobes of the thymus upward and outward, producing a distinctive wedge-shaped shadow called the “spinnaker-sail sign” on a frontal chest X-ray. The sharp lower border of the elevated thymus, outlined by translucent air beneath it, resembles the sail of a boat catching wind.25PubMed Central. The Spinnaker-Sail Sign: Neonatal Pneumomediastinum Recognizing this sign prevents misidentifying mediastinal air as a mass or consolidation and ensures the baby gets appropriate monitoring for a possible concurrent pneumothorax.
What to Watch for After Treatment
Once a pneumothorax has been drained, follow-up X-rays serve a different purpose: confirming lung re-expansion and watching for complications. The most feared post-treatment complication visible on imaging is re-expansion pulmonary edema (REPE). This occurs when a lung that has been collapsed re-inflates and develops fluid-filled alveoli on the affected side. On a chest X-ray, REPE shows up as a unilateral hazy pattern on the treated side, typically appearing within two to four hours of re-expansion. The findings tend to worsen over the first 48 hours, persist for several days, and then gradually clear over about five to seven days.26PubMed Central. Severe re-expansion pulmonary edema after chest tube insertion for the treatment of spontaneous pneumothorax: A case report CT imaging in REPE cases shows ground-glass opacities, thickened tissue between the lobes, and sometimes areas of dense consolidation on the same side. Recognizing REPE early matters because it can cause significant respiratory distress and, in severe cases, requires intensive support.