Bloody Pleural Effusion With Only Two Stains Left
A 64-year-old man with a 40 pack-year smoking history has a recurrent right pleural effusion. A 150 mL sample arrives grossly bloody. Two cytospin preparations show cohesive three-dimensional clusters of cells with high nuclear-to-cytoplasmic ratios, prominent nucleoli, and vacuolated cytoplasm against a background of small lymphocytes and reactive mesothelial cells. The cell block is a fibrin clot containing roughly 30 cells - too few for staining. Immunostains performed on the cytospin preparations show MOC-31 and Ber-EP4 positive, calretinin and CK5/6 negative, and WT1 negative. TTF-1 is weak and equivocal; the blood and protein background makes the result difficult to interpret. The laboratory has material for exactly two additional stains.
1. Can you render a definitive diagnosis, and how does the inadequate cell block affect how you word the report?
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Diagnosis: positive for malignant cells, consistent with adenocarcinoma, metastatic.
- The morphology (three-dimensional clusters, high nuclear-to-cytoplasmic ratio, macronucleoli, cytoplasmic vacuoles) plus a carcinoma immunoprofile (MOC-31 and Ber-EP4 positive; calretinin, CK5/6, WT1 negative) is sufficient on a cytospin preparation.
- Immunostains on cytospin preparations are acceptable provided they are validated for that matrix and appropriate controls are run; the matrix does not have to be a cell block.
- The 30-cell cell block is not adequate and should not be used for stains or for ancillary molecular testing. Say so in the report rather than implying a block was evaluated - otherwise the clinician may order PD-L1 or EGFR testing on a specimen that cannot support it.
- Do not downgrade to 'suspicious' simply because the block failed. Suspicious is for cases where the morphology itself is equivocal; here the morphology is unequivocal and only the block is inadequate. Use a comment: 'Cell block inadequate for ancillary studies; recommend repeat thoracentesis if molecular profiling is required.'
2. Choose the two remaining stains and justify the ranking against the clinical question.
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Choose Napsin A and p40 (or p63).
Ranking logic - what is the oncologist actually going to do with the answer?
- TTF-1 was uninterpretable, so the lung origin is unproven. Napsin A is the highest-yield replacement: it is more specific than TTF-1 for lung adenocarcinoma and its cytoplasmic pattern is less affected by the proteinaceous background that confounded the TTF-1. A positive Napsin A with the existing Ber-EP4/MOC-31 profile supports a lung primary.
- Squamous cell carcinoma is the competing thoracic diagnosis and drives a completely different workup (no driver-mutation testing, different systemic therapy). p40 is the most specific squamous marker; a negative p40 supports adenocarcinoma.
- Stains I would not spend here: more mesothelial markers (calretinin, CK5/6, WT1 already negative and the differential with mesothelioma is settled), and proliferation or 'screening' panels, which do not change management.
Caveat: weak TTF-1 in a bloody background should be called uninterpretable, not negative or weak-positive; if a stain cannot be interpreted with a clean internal control, it must be repeated or disregarded.
3. The oncology team asks what to do next to enable molecular testing. What do you recommend, and what are the limits?
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Recommend a repeat, larger-volume thoracentesis with a dedicated cell block, ideally 200-500 mL, and mention pleural biopsy as the alternative if the repeat is also acellular.
- Fluid cytology sensitivity for malignancy is on the order of 60-70% per sample; a single sample misses a substantial minority, and a negative repeat does not exclude malignancy. Sending a second specimen is therefore reasonable and not redundant.
- Molecular testing (PD-L1, EGFR, ALK, etc.) requires a block with adequate tumor cellularity - commonly 100 or more viable tumor cells - which a 30-cell clot cannot provide. This is the real bottleneck, not the diagnosis.
- Tell the clinician the diagnosis is already secure; the repeat is for tissue, so the request should specify cell block and molecular studies so the fluid is not discarded after smears.
- If a second tap fails, image-guided pleural biopsy is the next step and should be requested before treatment decisions are delayed.