A 72-year-old man is evaluated in the hospital prior to discharge. He was admitted five da… — USMLE Step 3

A $72$ -year-old man is evaluated in the hospital prior to discharge. He was admitted five days ago for an acute exacerbation of heart failure with reduced ejection fraction, presenting with severe dyspnea and bilateral pleural effusions. Over the hospital course, he has been treated aggressively with intravenous furosemide, resulting in a net fluid output of $6.5\text{ L}$ and significant improvement in his symptoms. A follow-up chest radiograph shows resolution of the left pleural effusion but persistence of a moderate right-sided pleural effusion. To rule out other causes before discharge, a diagnostic thoracentesis is performed.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Serum
Total protein	$7.6\text{ g/dL}$	$6.0-8.0\text{ g/dL}$
Albumin	$4.1\text{ g/dL}$	$3.5-5.0\text{ g/dL}$
Lactate dehydrogenase (LDH)	$150\text{ U/L}$	$100-250\text{ U/L}$
Pleural Fluid
Total protein	$4.6\text{ g/dL}$	-
Albumin	$2.5\text{ g/dL}$	-
Lactate dehydrogenase (LDH)	$110\text{ U/L}$	-
Glucose	$90\text{ mg/dL}$	-
pH	$7.45$	-
Cell count	$250/\mu\text{L}$ (predominantly mononuclear)	-

Based on the laboratory findings, which of the following is the most likely etiology of this patient's pleural effusion?

Congestive heart failureAnswer
B
Pulmonary embolism
C
Parapneumonic effusion
D
Malignant pleural effusion
E
Tuberculous pleuritis

Answer

Congestive heart failure

The correct diagnosis is congestive heart failure. The patient's pleural fluid initially meets Light's criteria for an exudate because the pleural fluid-to-serum protein ratio is

> 0.5

(calculated as

4.6 / 7.6 = 0.605

) and the LDH ratio is

> 0.6

(calculated as

110 / 150 = 0.733

). However, in patients with known heart failure who undergo aggressive diuresis, the pleural fluid can concentrate as water is removed, resulting in a 'pseudoexudate'. When clinical suspicion for a transudate remains high despite exudative Light's criteria, the serum-to-pleural fluid albumin gradient must be calculated. A gradient

> 1.2\text{ g/dL}

indicates a transudative etiology. This patient's gradient is

1.6\text{ g/dL}

(

4.1 - 2.5

), which confirms a transudate caused by congestive heart failure.

Step-by-Step Solution

Calculate Light's criteria for the pleural fluid.

The pleural fluid-to-serum protein ratio is

0.605

(

4.6 / 7.6

) and the LDH ratio is

0.733

(

110 / 150

). Both exceed the thresholds (

>0.5

for protein,

>0.6

for LDH), classifying the fluid as an exudate.

This is the standard initial step for evaluating the etiology of any pleural effusion.

Incorporate the clinical context of aggressive diuresis.

Recognize that the patient has heart failure and recently lost

6.5\text{ L}

of fluid from diuresis, which can concentrate pleural fluid and create a 'pseudoexudate'.

To avoid prematurely misdiagnosing a transudate as an exudate and pursuing unnecessary, invasive, or expensive diagnostic workups.

Calculate the serum-to-pleural fluid albumin gradient.

Subtract pleural fluid albumin from serum albumin:

4.1 - 2.5 = 1.6\text{ g/dL}

An albumin gradient

> 1.2\text{ g/dL}

accurately identifies a transudate in patients receiving diuretic therapy, effectively overriding exudative Light's criteria.

Key Concept

Differentiating a true exudate from a diuretic-induced pseudoexudate using the serum-to-pleural fluid albumin gradient.

Estimated Time:2m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic