Respiratory Investigation — Written Clinical Reasoning

Model answer — Part A: three PFT panels

Patient 1 — confirmed parenchymal restriction. The TLC below the lower limit of normal confirms true restriction (spirometry alone could only suggest it), and the DLCO of 52% localises the defect to the alveolar-capillary membrane: this is the interstitial lung disease pattern. The single most discriminating result is the DLCO — it is what separates restriction within the lung from restriction outside it. The next investigation is HRCT chest to define the pattern (UIP versus non-UIP), which drives the whole subsequent pathway [1] [2].

Patient 2 — no restriction: the obesity mimic. The low FVC with preserved ratio suggested restriction on spirometry, but the TLC of 88% refutes it — this patient has no ventilatory defect, and the normal DLCO confirms an intact membrane. The most discriminating result is the TLC, which is exactly why volumes must precede the word "restriction". The next step is not more lung testing but an explanation for dyspnoea: obesity-related mechanics, deconditioning, and clinical assessment for cardiac or vascular causes if symptoms warrant [1] [3].

Patient 3 — neuromuscular restriction. TLC confirms restriction, but the normal DLCO with a normal-to-high KCO says the membrane is intact — the problem is the pump, not the lung. The most discriminating result is the combination of normal DLCO with a 25% sitting-to-supine FVC fall, pointing at diaphragmatic weakness. The next investigations are maximal inspiratory and expiratory pressures and a sniff nasal inspiratory pressure (with diaphragm ultrasound), and a clinical search for motor neurone disease, myopathy or phrenic nerve pathology [1] [2].

The unifying mark-scorer sentence: restriction is a TLC diagnosis; DLCO (with KCO) then splits parenchymal from extrapulmonary causes; and obesity is the classic pseudo-restriction that only volumes can dismiss [1].

Model answer — Part B: unilateral exudative effusion

Interpretation. The fluid is an unequivocal exudate by Light's criteria — all three are met (protein ratio 0.63, LDH ratio 1.8, fluid LDH above two-thirds of the serum ULN). A persistent unilateral exudate with weight loss is malignancy until excluded; one negative cytology does not end the workup, because cytology misses a substantial minority of malignant effusions [4]. (If he were heavily diuresed for heart failure, I would guard against the pseudoexudate trap with a serum–pleural protein gradient over 3.1 g/dL — but nothing in this vignette suggests diuresis, and the gradient here is only 2.6 g/dL.) [5]

Sequence of workup. First, contrast-enhanced CT chest (before complete drainage, so the pleura can be assessed) to look for nodular pleural thickening, a parenchymal lesion or nodes, and to map any biopsy target. Second, proceed to pleural tissue — cytology has had its chance. Third, if histology is malignant, integrate staging and molecular workup; if it shows non-specific inflammation, escalate rather than repeat [6] [7].

Biopsy-route reasoning. Choose by target and by question. If CT shows focal pleural thickening or nodules, image-guided cutting-needle biopsy is a high-yield day-case route. If CT shows no target, or the effusion is free-flowing with cytology negative, medical thoracoscopy gives direct vision, large targeted biopsies and the option of talc pleurodesis at the same sitting — the randomised Metintas comparison found both routes highly sensitive, so the answer is fit-to-pleura, not hierarchy. Blind Abram's biopsy is the fallback only where neither image guidance nor thoracoscopy is available, and VATS is reserved for surgical pathology or failed lung. All sampling should be ultrasound-or-image planned, per BTS pleural-procedure guidance, and in suspected mesothelioma the access sites are documented because tracts can seed [6] [7].