It is about interpretation of results of pleural fluid tests including physical characteristics, biochemical analysis, cell counts, culture and cytology.

2. • Thoracentesis (pleural tapping) should be
performed for new and unexplained pleural
effusions.

3. Certain types of pleural effusions can be suspected
simply by observing the physical characteristics of
the fluid obtained:
 A purulent fluid (pus) indicates an empyema
 A bad (putrid) odor suggests an anaerobic empyema
 A milky white fluid suggests a chylothorax (an
accumulation of lymphatic fluid in the pleural
space)

4.  A bloody fluid suggests either
 hemorrhagic effusion (hemothorax) or
 traumatic pleural taps.
Differentiation is possible by observing serial samples of
pleural tap which clear up in the case of a traumatic pleural
tap.
Moreover, A hematocrit performed on the pleural fluid can
assist diagnosis.
Bloody pleural fluid with a hematocrit of ≥ 50 % of the
peripheral blood hematocrit is termed a hemothorax.

5.  A Black pleural fluid can be seen with some diseases
including:
• Aspergillus niger infection,
• malignant melanoma,
• non-small cell lung cancer,
• ruptured pancreatic pseudocyst,
• charcoal-containing empyema
CONT.

6. Normal pleural fluid is a Clear ultrafiltrate of plasma that
originates from the parietal pleura:
 pH of 7.60-7.64
 Protein content of less than 2% (1-2 g/dL)
 Fewer than 1000 white blood cells (WBCs) per cubic
millimeter
 Glucose content similar to that of plasma
 Lactate dehydrogenase (LDH) less than 50% of plasma

7. Light’s criteria are the standard way to
differentiate.
Light’s criteria for exudative pleural effusion:
 Pleural fluid protein divided by serum protein > 0.5
 Pleural fluid LDH divided by serum LDH > 0.6
 Pleural fluid LDH more than two-thirds the upper
limit of normal serum LDH

8. The fluid is considered an exudate if any of the three
criteria (Light’s criteria) is found.
The fluid is considered a transudate if all of the three
criteria are absent.
Light’s criteria require simultaneous measurement of
pleural fluid and serum protein and LDH.
Clinical judgment is required when pleural fluid test
results fall near the cutoff points.

9. Light’s criteria identify nearly all exudates correctly, but they
misclassify approximately 20-25% of transudates as exudates,
usually in patients with HF-associated effusions and those with
liver cirrhosis-associated effusions because of the
concentrating effect of long-term diuretic therapy on protein
and LDH levels within the pleural space).

10. To solve such a problem it is recommended to use one of the following If the
clinical picture is consistent with HF-associated effusion or liver cirrhosis-
associated effusion but the pleural fluid meets Light's exudative criteria:
1. - serum-effusion protein gradient (serum protein minus pleural protein
concentration). If >3.1 g/dL, it indicates transudate effusion.(5)
2. - serum-effusion albumin gradient (serum albumin minus pleural effusion
albumin level). If it is > 1.2 g/dL, it indicates transudate. (this criterion
is preferable If the clinical picture is consistent with HF but the pleural
fluid meets Light's exudative criteria). (5)
3. - effusion-to-serum albumin ratio (pleural fluid albumin divided by serum
albumin). If <0.6, it indicates transudate. this criterion is preferable In
the context of cirrhosis. (5)

11. In a more recent systematic review,
• pleural fluid cholesterol greater than 55 mg/dL and
• pleural fluid LDH greater than 200 U/L
each had better positive and negative likelihood ratio for
distinguishing exudates from transudates than did Light’s
criteria. [4]

12. Pleural fluid LDH
 Pleural fluid LDH levels › 1000 IU/L suggest:
empyema, malignant effusion, rheumatoid effusion.
 Pleural fluid LDH levels are also increased in effusions from
Pneumocystis jiroveci (formerly, Pneumocystis carinii)
pneumonia.
The diagnosis of Pneumocystis jiroveci pneumonia is suggested
by: - a pleural fluid/serum LDH ratio of › 1, with
- a pleural fluid/serum protein ratio of ‹ 0.5.

13. Pleural fluid glucose
 A low pleural glucose concentration (30-50 mg/dL)
suggests malignant effusion, tuberculous pleuritis,
esophageal rupture, or lupus pleuritis.
 A very low pleural glucose concentration (ie, < 30
mg/dL) further restricts diagnostic possibilities, to
rheumatoid pleurisy or empyema.

14. Pleural fluid pH
Pleural fluid pH is highly correlated with pleural fluid
glucose levels.
A pleural fluid pH of less than 7.30 with a normal
arterial blood pH level is caused by the same diagnoses
listed for low pleural fluid glucose.

15. Pleural fluid pH
For parapneumonic effusions, a low pleural fluid pH
level is predictive of complicated effusions (that
require drainage) .
In such cases, a pleural fluid pH of less than 7.1-7.2
indicates the need for urgent drainage of the effusion,
while a pleural fluid pH of more than 7.3 suggests that
the effusion may be managed with systemic antibiotics
alone.

16. If an exudate is suspected clinically or is confirmed by
chemistry test results, send the pleural fluid for :
 total and differential cell counts,
 Gram stain and culture,
 cytology.

17. Pleural fluid lymphocytosis, with lymphocyte values greater than
85% of the total nucleated cells, suggests:
 TB,
 lymphoma,
 Chylothorax,
 sarcoidosis,
 chronic rheumatoid pleurisy,
 yellow nail syndrome.
Pleural lymphocyte values of 50-70% of the nucleated cells
suggest malignancy.

18.  Cultures of infected pleural fluids yield positive
results in approximately 60% of cases.
 This occurs even less often for anaerobic
organisms.
 Diagnostic yields, particularly for anaerobic
pathogens, may be increased by directly culturing
pleural fluid into blood culture bottles.

19.  Malignancy is suspected in patients with
lymphocytic exudative effusions, especially when
bloody.
 Direct tumor involvement of the pleura is diagnosed
most easily by performing pleural fluid cytology.
 The reported diagnostic yields in cytology vary
from 60-90%, depending on the extent of pleural
involvement and the type of primary malignancy.

20. Suspect tuberculous pleuritis in patients with
lymphocytic exudative effusions, especially if less
than 5% mesothelial cells are detected on
differential cell counts.

21. Most tuberculous pleural effusions probably result from a
hypersensitivity reaction to the Mycobacterium rather
than from microbial invasion of the pleura. So that,
 acid-fast bacillus stains of pleural fluid are rarely
diagnostic (< 10% of cases), and
 Pleural fluid cultures grow M tuberculosis in < 65% of
cases.

22. pleural biopsy:
The combination of histology and culture of pleural
tissue obtained by pleural biopsy increases the
diagnostic yield for TB to 90%.

23. Adenosine deaminase (ADA) activity of greater than
50 U/mL in pleural fluid supports the diagnosis of
tuberculous pleuritis.
However, pleural ADA values of less than 50 U/mL
do not exclude the diagnosis of TB pleuritis.

24.  Exudate,
 lymphocytic predominance,
 positive acid-fast bacillus smear or cultures,
 ADA > 50 U/L

25.  Exudative with PMN predominance/pus,
 positive Gram stains or cultures,
 LDH > 1000,
 glucose < 40 mg%,
 pH < 7.2
PMNs = polymorph nuclear leukocytes, which are a
special family of white blood cells that ncludes
neutrophils, eosinophils, and basophils.

26.  Black-colored,
 fungal smear, culture positive

27.  Exudate,
 lymphocytic predominance,
 positive cytology

28.  Hemorrhagic,
 hematocrit > 50% of blood

29.  Triglycerides > 110 mg/dL,
 chylomicrons,
 cholesterol/triglyceride ratio < 1

30.  Exudate,
 lymphocytic predominance,
 rheumatoid factor positive > 1:320,
 low glucose < 40 mg%,
 ADA > 50 U/L

31.  Exudate with PMN predominance,
 LE cells positive,
 ANA positive > 1:160

32.  pH < 7,
 high salivary amylase

33. 1. https://emedicine.medscape.com/article/299959-workup
2. https://www.pulmonologyadvisor.com/home/decision-support-
in-medicine/pulmonary-medicine/hemorrhagic-pleural-
effusions-and-hemothorax/
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4753987/
4. Wilcox ME, Chong CA, Stanbrook MB, Tricco AC, Wong C,
Straus SE. Does this patient have an exudative pleural
effusion? The Rational Clinical Examination systematic
review. JAMA. 2014 Jun 18. 311(23):2422-31. [Medline].
5. https://www.ncbi.nlm.nih.gov/pubmed/22372660