Review Question - QID 217095

This patient with progressive dyspnea, pleuritic chest pain, nonproductive cough, recent immigration from a country where tuberculosis (TB) is endemic, fever, tachypnea, right-sided pleural effusion, and sputum sample positive for acid-fast bacilli most likely has a tuberculous effusion. Tuberculous effusions are exudative by the Light criteria (elevated total protein, elevated LDH), usually show lymphocytosis (elevated nucleated cell count), and have elevated adenosine deaminase levels.

Pleural effusions are characterized as either transudative or exudative based on the Light criteria. This distinction can help elucidate the etiology of a pleural effusion. A patient is considered to have an exudative effusion if the pleural fluid (PF) to serum protein ratio ≥ 0.5, PF to serum lactate dehydrogenase (LD) ratio ≥ 0.6, or a PF LD ≥ 2/3rd of the upper limit of normal. Otherwise, the patient has a transudative effusion. Infections lead to cytokine release and increased capillary permeability. This in turn leads to the translocation of fluid and cells into the pleural space, which both aids in the immune response and results in an exudative effusion. In particular, tuberculous effusions are usually exudative, right-sided, show moderate lymphocytosis, very elevated protein levels, low pleural pH, and low pleural glucose levels. In addition, tuberculous effusions often show elevated levels of adenosine deaminase, an enzyme produced by lymphocytes and involved in purine metabolism.

Feller-Kopman and Light (namesake of the Light criteria) review the pathophysiology, common causes, and evaluation of transudative and exudative pleural effusions. This paper also discusses the shortcomings of the Light criteria, noting that while these criteria are excellent at identifying exudates, up to 25% of transudative effusions may be misclassified.

Figure A presents the possible answers choices for this question. Values for total protein, lactate dehydrogenase (LDH), nucleated cell count, and adenosine deaminase are shown in the table.

Incorrect Answers:
Answer 1: This answer choice is mostly consistent with a transudative effusion (fluid/serum protein ratio of 0.4/6.2 ~ 0.006 < 0.5, fluid/serum LDH ratio of 40/160 ~ 0.25 < 0.6, and fluid LDH < 2/3rd upper limit of normal). While tuberculous effusions would be expected to have elevated adenosine deaminase levels, they are always exudative, not transudative.

Answer 2: This answer choice is consistent with a transudative effusion (fluid/serum protein ratio of 0.4/6.2 ~ 0.006 < 0.5, fluid/serum LDH ratio of 40/160 ~ 0.25 < 0.6, and fluid LDH < 2/3rd upper limit of normal). Transudative effusions are caused by the movement of fluid from the vasculature into the pleural space and may result from heart failure, cirrhosis, nephrotic syndrome, or pericardial disease.

Answer 3: This answer choice is consistent with an exudative effusion (fluid/serum protein ratio > 0.5, fluid/serum LDH ratio > 0.6, fluid LDH > 2/3rd upper limit of normal). However, in a tuberculous effusion, adenosine deaminase levels and nucleated cell count would be expected to be elevated. This answer choice could be consistent with a chylothorax.

Answer 5: This answer choice is consistent with an exudative effusion (fluid/serum protein ratio > 0.5, fluid/serum LDH ratio > 0.6, fluid LDH > 2/3rd upper limit of normal) with elevated nucleated cell count. However, in tuberculous effusions, adenosine deaminase is usually elevated. This answer choice could be consistent with many other etiologies of exudative effusions including malignancy, infectious pneumonia, esophageal rupture, and collagen vascular disease.

Bullet Summary:
Tuberculous effusions are exudative by the Light criteria (elevated fluid/serum protein ratio > 0.5, fluid/serum LDH > 0.6, fluid LDH > 2/3rd upper limit of normal), show moderate lymphocytosis, and have elevated adenosine deaminase levels.