How pleural effusions are tested on Step 1

Questions give pleural fluid and serum values and ask whether the fluid is a transudate or exudate and what the cause is, or describe a pneumonia that fails to improve.

workspace_premium One criterion is enough

Light criteria classify fluid as an exudate if any one of the three criteria is met. They are very sensitive for exudates, so diuresed heart failure patients can be misclassified.

Transudate vs exudate

Transudates come from pressure problems across intact membranes; exudates from leaky, inflamed, or obstructed pleura.

Transudate↑ Hydrostatic or ↓ oncotic pressureProtein ratio ≤ 0.5, LDH ratio ≤ 0.6Heart failure (most common overall)Cirrhosis, nephrotic syndromeTreat the underlying conditionExudate↑ Capillary permeability or ↓ lymphdrainageAny Light criterion metPneumonia, malignancy, TBPE, pancreatitis, rheumatoid arthritis,SLEOften needs drainage or further workupvs
Transudate vs exudate. PE can produce either type, though usually an exudate.
Light criterion (exudate if any)Threshold
Pleural protein ÷ serum protein> 0.5
Pleural LDH ÷ serum LDH> 0.6
Pleural LDH> ⅔ of the upper limit of normal serum LDH

If heart failure is suspected but Light criteria suggest exudate after diuresis, a serum–pleural albumin gradient > 1.2 g/dL supports a transudate.

Exudate workup and special effusions

Additional fluid tests point to the cause.

Pleural fluid cluesLow glucoseEmpyema, RA,TB, malignancyLow pH <7.2Empyema — drainHighamylasePancreatitis,esophagealruptureHigh ADATuberculosis(lymphocytic)Milky, ↑ TGChylothorax
Fluid clues. Cytology diagnoses malignant effusions in about 60%; repeat sampling or pleural biopsy raises the yield.
EffusionFeaturesManagement
Uncomplicated parapneumonicFree-flowing, pH ≥ 7.2, glucose normalAntibiotics alone
Complicated parapneumonic / empyemapH < 7.2, glucose < 60, positive culture, loculated, or pusAntibiotics + chest tube; fibrinolytics or VATS if loculated
MalignantExudate, positive cytology; lung, breast cancer, lymphoma, mesotheliomaTherapeutic thoracentesis, pleurodesis, indwelling catheter
HemothoraxPleural hematocrit > 50% of blood hematocritChest tube; surgery if ongoing bleeding
ChylothoraxMilky, triglycerides > 110 mg/dL, chylomicronsDrainage, low-fat diet, duct ligation

Rheumatoid effusions classically have very low glucose; esophageal rupture causes high amylase and low pH.

Pathophysiology and diagnosis

Normally about 10–20 mL of fluid lubricates the pleural space, with filtration from parietal capillaries balanced by lymphatic drainage.

DetectExam, chestx-ray,ultrasoundTap thefluidThoracentesisunless clearHFLightcriteriaTransudate orexudateExudatetestsCell count,glucose, pH,cytology,cultureTreatcauseDrain ifempyema
Diagnostic approach. Blunting of the costophrenic angle on an upright film requires about 200 mL of fluid.

Clinical application

A pneumonia that does not improve suggests a complicated effusion.

functions Worked example — the persistent fever

A patient treated for 4 days for pneumococcal pneumonia still has fevers. Ultrasound shows a loculated effusion. Fluid: protein ratio 0.7, LDH ratio 0.9, pH 7.05, glucose 38 mg/dL, Gram-positive diplococci. What is the classification and management?

  1. Protein and LDH ratios exceed Light thresholds — exudate.
  2. pH below 7.2, low glucose, and organisms on Gram stain indicate a complicated parapneumonic effusion (empyema).
  3. Antibiotics cannot sterilize an undrained, loculated collection.

Answer. Empyema. Continue antibiotics and place a chest tube; give intrapleural fibrinolytic therapy or proceed to VATS if loculations prevent drainage.