urinothorax is associated with obstructive uropathy after trauma or surgery
- related: pleural effusion
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Differentiating transudative from exudative effusions is useful in two ways: The differential diagnosis for transudates is much shorter, and the fluid itself is rarely problematic unless large in volume. This patient's transudate does not have an obvious cause, but there is a clue that some of the data may be incorrect. Repeating a thoracentesis and sending the sample for several unique tests would confirm the diagnosis, which in this case is caused by urine (choice C is correct). Urinothorax is an uncommon cause of transudative pleural effusions and is usually associated with obstructive uropathies but can also develop after trauma or be iatrogenic. It is typically characterized as meeting transudative criteria but with a ratio of pleural to serum creatinin >1.0, along with a low pleural pH. These criteria can make the diagnosis, and technetium 99m renal scans can identify the source of the urine leak. Checking pleural creatinine is not a standard test on pleural effusions but would be helpful in this situation. Pleural pH is more commonly measured but often processed improperly. This patient's pleural pH of 7.51 suggests either a profound metabolic alkalosis not represented on his serum chemistry panel or an inaccurate measurement. The latter is more likely because pleural fluid sent for pH requires the sample be stored anaerobically and processed within 4 h. Samples are often stored in capped vials containing significant amounts of air. Delays in processing compounded by aerobic storage methods can dramatically raise pH levels, providing misleading information. Historically, storing pleural fluid samples on ice was recommended, but recent studies suggest this is unnecessary. Of note, both lidocaine and heparin in storage tubes can lower the fluid's pH. A repeat sample processed correctly would reveal a low pH and an opportunity to measure the pleural creatinine level. Another possibility for his transudate is a leak of cerebrospinal fluid resulting from his earlier traumatic injury. The presence of β-2-transferrin in pleural fluid is highly suggestive of a duropleural fistula.1
42-year-old man is referred for further evaluation of a recurrent pleural effusion. He reports no history of breathing difficulties or respiratory health problems but has noticed dyspnea in the past 4 months for which he sought medical attention. He reports he had a collection of fluid around his right lung that was sampled last month and that it was “clear.” He brings a copy of the test results (Figure 1) and a copy of a recent chest radiograph demonstrating a moderate right pleural effusion but otherwise normal. He reports no fevers, chills, sweats, orthopnea, or lower extremity swelling. The patient reports being in good health except for injuries sustained during a car crash last year that involved several orthopedic and abdominal surgeries. He reports no chronic health conditions and takes no medications. He reports drinking one or two beers on weekends, claims to be a lifelong nonsmoker, and reports no recreational drug use. He is sexually active with one partner for the past 16 years.
On physical examination, the patient is alert and oriented to person, place, and time. His vital signs are as follows: BP, 124/70 mm Hg; heart rate, 74/min; respiratory rate, 12/min; SpO2, 97% on room air; temperature, 36.8°C; and BMI, 26 kg/m2. There are no jaundice or skin rashes. The nasal/oropharynx examination is normal. There are no neck masses, jugular venous distension, or palpable lymph nodes. His chest is normal to inspection, with a dull right base to auscultation and percussion but no adventitious sounds. There are no heart murmurs, rubs, or gallops. There is a large ventral abdominal scar that is well healed, with active bowel sounds and without guarding or rebound. There is no palpable organomegaly. There are several scars on the extremities from external fixators, which are warm and dry, with no edema. Dorsalis pedis pulses bilaterally are strong.