Question 3.4

College answer

Not available.

Discussion

a) This is some sort of horrible blast crisis due to a haematological malignancy. From the presented data it would be impossible to say what kind of malignancy with any precision, apart from narrowing it down to some sort of leukaemia. Both acute and chronic versions can cause hyperviscosity but the chronic ones typically have a more insidious onset, resulting in a higher white cell count and more risk of hyperviscosity, whereas the acute ones often do not usually get the chance to become hyperviscous.

b) "What other organ is most likely to be affected by this phenomenon" is a strange question to ask in this top-tier elite exit exam, as it is not clear what it's testing apart from basic reading comprehension. The stem clearly gave us seizures and coma. This blast crisis patient probably has some kind of leukostasis-related complication related to an increase in their blood viscosity (most likely either an ischaemic stroke or a cerebral venous sinus thrombosis). If the reader interpreted the question as asking "other than the brain, what other organ is most likely to be affected by this process?" then the answer could include:

• Lungs (hypoxia due to increased blood viscosity, pulmonary hypertension due to increased resistance to flow),
• Eyes (retinal bleeding, retinal vein thrombosis)
• Myocardium (ischaemia in coronary arteries the stenosis of which was until recently subclinical and unimportant, due to a combination of increased flow resistance and the increased myocardial workload of pumping all that super-viscous blood)
• Kidneys (renal vein thrombosis)
• Penis (priapism)

c) This is "leukocyte larceny", spurious hypoxemia on the ABG which is the consequence of ongoing metabolism of the oxygen by the blasts in the sample. One must nod approvingly at the college examiners who introduced a sample processing delay into the question ("arrives from the lab" they mentioned quietly, i.e. it was not analysed by a locally available point-of-care device which would usually eliminate this phenomenon). While the intern was running the blood gas to pathology, the blasts in the syringe were misappropriating all the oxygen like Russian oligarchs.  The result was a falsely depressed PaO₂ in the sample. The lab would not have to be very far away: according to the data collected by Fox et al, the rate of metabolism can be very rapid. For blood samples with the highest WCC counts, the investigators observed a PaO₂ drop of up to 72 mmHg over two minutes.