A 58-year-old man has had right lower lobe pneumonia for 6 days, with fever persisting despite antibiotics. A right pleural effusion is tapped; the fluid is cloudy but not frank pus. Pleural fluid: protein 42 g/L, LD 1,350 U/L, glucose 1.9 mmol/L (34 mg/dL), pH 7.08 measured at once on a blood gas analyzer from a heparinized syringe with no air or local anesthetic; Gram stain negative. Serum: protein 70 g/L, LD 260 U/L (upper reference limit 250 U/L), glucose 6.4 mmol/L (115 mg/dL). The chest team drains parapneumonic effusions with a pH below 7.20. The best interpretation is:

Exudate; complicated effusion needing drainage

Exudate; pH unreliable, repeat the tap

Transudate; no further fluid tests needed

Exudate; uncomplicated, antibiotics alone

A 61-year-old woman with long-standing seropositive rheumatoid arthritis has a right pleural effusion. She is afebrile with no pneumonia on imaging. The fluid is an exudate with glucose 0.9 mmol/L (16 mg/dL), plasma glucose 5.8 mmol/L (104 mg/dL), pH 7.12, negative Gram stain and culture, ADA 15 U/L (cut-off 40 U/L) and no malignant cells. The most likely cause is:

Complicated parapneumonic effusion

A ward sends pleural fluid for protein and LD, asking whether it is a transudate or an exudate. What else does the laboratory need to answer the question?

Serum collected close to the time of the tap

A second pleural sample taken the next day

A 24-hour urine for total protein

Serous fluids

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Serous fluids, part of Urinalysis and Body Fluids. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 58-year-old man has had right lower lobe pneumonia for 6 days, with fever persisting despite antibiotics. A right pleural effusion is tapped; the fluid is cloudy but not frank pus. Pleural fluid: protein 42 g/L, LD 1,350 U/L, glucose 1.9 mmol/L (34 mg/dL), pH 7.08 measured at once on a blood gas analyzer from a heparinized syringe with no air or local anesthetic; Gram stain negative. Serum: protein 70 g/L, LD 260 U/L (upper reference limit 250 U/L), glucose 6.4 mmol/L (115 mg/dL). The chest team drains parapneumonic effusions with a pH below 7.20. The best interpretation is:
1. AExudate; pH unreliable, repeat the tap
2. BExudate; complicated effusion needing drainage
3. CTransudate; no further fluid tests needed
4. DExudate; uncomplicated, antibiotics alone
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Answer: B · Exudate; complicated effusion needing drainage
By Light's criteria the fluid is an exudate on all three counts: the protein ratio is 42/70 = 0.60 (above 0.5), the LD ratio is 1,350/260 = 5.2 (above 0.6), and the fluid LD is far above two-thirds of the serum upper limit (167 U/L). In a parapneumonic exudate, a pH below 7.20, a low glucose and a very high LD show that bacteria and neutrophils in the pleural space are making acid and using glucose. This is a complicated parapneumonic effusion, which needs chest tube drainage even though the fluid is not yet frank pus (empyema) and the Gram stain is negative. The pH is reliable because the fluid was collected without air or lidocaine and measured at once: air falsely raises the pH and lidocaine lowers it.
- A: The sample was anaerobic, without lidocaine and measured at once, so the pH is reliable.
- C: All three of Light's criteria are met, so the fluid is an exudate.
- D: A pH below 7.20 and a low glucose mark an effusion that antibiotics alone will not clear.
Question 2 · Hard
Serum protein is 70 g/L and serum LD is 300 U/L; the upper reference limit for serum LD is 240 U/L. Which pleural fluid result set is a transudate by Light's criteria?
1. AProtein 30 g/L, LD 170 U/L
2. BProtein 40 g/L, LD 110 U/L
3. CProtein 24 g/L, LD 120 U/L
4. DProtein 33 g/L, LD 190 U/L
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Answer: C · Protein 24 g/L, LD 120 U/L
Check each criterion: protein ratio above 0.5, LD ratio above 0.6, or fluid LD above two-thirds of 240, which is 160 U/L. Protein 24 g/L with LD 120 U/L gives ratios of 0.34 and 0.40 and an LD below 160 U/L, so it meets none and is a transudate. Each of the other sets meets at least one criterion. Because serum LD here is above its upper limit, the absolute LD rule catches a fluid that the LD ratio misses.
- A: Fluid LD of 170 U/L is above two-thirds of 240 U/L (160 U/L).
- B: The protein ratio is 40/70 = 0.57, above 0.5.
- D: The LD ratio is 190/300 = 0.63, and fluid LD is above 160 U/L.
Question 3 · Hard
A 61-year-old woman with long-standing seropositive rheumatoid arthritis has a right pleural effusion. She is afebrile with no pneumonia on imaging. The fluid is an exudate with glucose 0.9 mmol/L (16 mg/dL), plasma glucose 5.8 mmol/L (104 mg/dL), pH 7.12, negative Gram stain and culture, ADA 15 U/L (cut-off 40 U/L) and no malignant cells. The most likely cause is:
1. ATuberculous pleurisy
2. BComplicated parapneumonic effusion
3. CRheumatoid pleurisy
4. DMalignant pleural effusion
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Answer: C · Rheumatoid pleurisy
A very low pleural glucose, often below 1.7 mmol/L (30 mg/dL), with a low pH in a patient with rheumatoid arthritis is typical of rheumatoid pleurisy. The thickened, inflamed pleura limits glucose transport into the fluid while cells in the fluid keep using it. Empyema can give the same chemistry, so infection must be excluded, here by the absence of fever, pneumonia and organisms. A low ADA argues against tuberculosis and negative cytology against cancer, although one negative cytology does not exclude it.
- A: Tuberculous effusions usually have an ADA above the 40 U/L cut-off.
- B: No fever, pneumonia or organisms argues against an infected parapneumonic effusion.
- D: No malignant cells were seen, and the rheumatoid history explains the findings.
Question 4 · Medium
A ward sends pleural fluid for protein and LD, asking whether it is a transudate or an exudate. What else does the laboratory need to answer the question?
1. AAn EDTA blood sample for a CBC
2. BSerum collected close to the time of the tap
3. CA second pleural sample taken the next day
4. DA 24-hour urine for total protein
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Answer: B · Serum collected close to the time of the tap
Light's criteria are ratios of pleural fluid to serum protein and LD, and they also compare fluid LD with the serum upper reference limit. A serum sample collected at about the same time as the thoracentesis is therefore essential; without it only the absolute fluid values can be reported. Serum taken days apart may have changed, especially its LD, which makes the ratios misleading. Many laboratories add a comment asking for paired serum when fluid arrives alone.
- A: A CBC does not give the serum protein and LD that the ratios need.
- C: A second fluid sample still has nothing to compare against.
- D: Urine protein loss can explain a low serum albumin but does not classify the fluid.
Question 5 · Easy
Under Light's criteria, a pleural effusion is an exudate if its fluid-to-serum total protein ratio is greater than:
1. A0.5
2. B0.67
3. C1.1
4. D0.6
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Answer: A · 0.5
Light's criteria call a pleural fluid an exudate if any one of three findings is present: a fluid-to-serum protein ratio above 0.5, a fluid-to-serum LD ratio above 0.6, or a fluid LD above two-thirds of the upper reference limit for serum LD. A transudate meets none of them. The criteria pick up almost all exudates, at the cost of labeling some transudates as exudates. Serum must be drawn at about the same time as the tap for the ratios to be valid.
- B: Two-thirds (0.67) applies to fluid LD against the upper reference limit of serum LD.
- C: 1.1 g/dL (11 g/L) is the serum-ascites albumin gradient cut-off used for ascites.
- D: 0.6 is the cut-off for the LD ratio, not the protein ratio.
Question 6 · Hard
A 76-year-old man with heart failure had 3 days of intravenous furosemide before a pleural tap. Fluid: protein 34 g/L, LD 120 U/L, albumin 18 g/L. Serum: protein 64 g/L, LD 230 U/L (upper reference limit 250 U/L), albumin 33 g/L. A serum-minus-fluid albumin gradient above 12 g/L (1.2 g/dL) favors a transudate. The best interpretation is:
1. AExudate; fluid LD is over half of serum LD
2. BIndeterminate; the tap must be repeated
3. CExudate; the protein ratio is above 0.5
4. DTransudate; diuresis raised the protein ratio
Answer and explanation in LearnFree account
Question 7 · Easy
A pleural effusion that meets none of Light's criteria is most often caused by:
1. ABacterial pneumonia
2. BTuberculous pleurisy
3. CMetastatic carcinoma
4. DCongestive heart failure
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Question 8 · Medium
A long-standing pleural effusion is milky with a satin-like sheen. Triglyceride is 0.5 mmol/L (44 mg/dL), cholesterol is 7.2 mmol/L (278 mg/dL) and cholesterol crystals are seen. The laboratory's chylothorax cut-off is a triglyceride above 1.24 mmol/L (110 mg/dL). The fluid is best described as:
1. ATrue chylous effusion
2. BPurulent empyema
3. CPseudochylous effusion
4. DHemorrhagic effusion
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Question 9 · Medium
A pleural fluid has a fluid-to-serum protein ratio of 0.42 and a fluid-to-serum LD ratio of 0.71. Fluid LD is below two-thirds of the serum upper reference limit. How is it classified by Light's criteria?
1. ATransudate, as fluid LD is below the limit
2. BTransudate, as the protein ratio is normal
3. CExudate, as one criterion is met
4. DIndeterminate, as two criteria are unmet
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Question 10 · Medium
A 48-year-old man has a left-sided pleural exudate. The pleural fluid amylase is several times higher than his serum amylase. Which condition best explains this?
1. ARheumatoid pleurisy
2. BTuberculous pleurisy
3. CPulmonary embolism
4. DAcute pancreatitis
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Serous fluids

Serum protein is 70 g/L and serum LD is 300 U/L; the upper reference limit for serum LD is 240 U/L. Which pleural fluid result set is a transudate by Light's criteria?

A ward sends pleural fluid for protein and LD, asking whether it is a transudate or an exudate. What else does the laboratory need to answer the question?

Under Light's criteria, a pleural effusion is an exudate if its fluid-to-serum total protein ratio is greater than:

A pleural effusion that meets none of Light's criteria is most often caused by:

A long-standing pleural effusion is milky with a satin-like sheen. Triglyceride is 0.5 mmol/L (44 mg/dL), cholesterol is 7.2 mmol/L (278 mg/dL) and cholesterol crystals are seen. The laboratory's chylothorax cut-off is a triglyceride above 1.24 mmol/L (110 mg/dL). The fluid is best described as:

A pleural fluid has a fluid-to-serum protein ratio of 0.42 and a fluid-to-serum LD ratio of 0.71. Fluid LD is below two-thirds of the serum upper reference limit. How is it classified by Light's criteria?

A 48-year-old man has a left-sided pleural exudate. The pleural fluid amylase is several times higher than his serum amylase. Which condition best explains this?