Question 3.4

A 54-year-old male presents with new onset confusion followed by seizures and convulsions, leading to intubation. His full blood count is shown:

Parameter	Patient Value	Adult Normal Range
Haemoglobin	105 g/L*	135 – 145
Mean Cell Volume	98 fL	80 – 100
Platelets	67 x 109/L*	150 – 400
White Cell Count	110 x 109/L*	4 – 11
Blasts	52%*	0

a) What is the likely cause of his presentation and underlying diagnosis?
b) What other organ is most likely to be affected by this phenomenon?
(15% marks)

Post intubation, the patient’s SpO2 holds steady at 98% on FiO2 25%. An arterial blood gas result arrives from the lab:

Parameter	Patient Value	Adult Normal Range
pH	7.31*	7.35 – 7.45
PaCO2	54 mmHg*	35 – 45
PaO2	45 mmHg*	80 – 100
SpO2	67%*	93 – 100

c) What is the likely cause of this discrepancy? (10% marks)

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College answer

Not available.

Discussion

a) This is some sort of horrible blast crisis due to an acute 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.

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.

References

Sacchetti, Alfred, et al. "Leukocyte larceny: spurious hypoxemia confirmed with pulse oximetry." The Journal of emergency medicine 8.5 (1990): 567-569.

Fox, Michael J., et al. "Leukocyte larceny: a cause of spurious hypoxemia." The American journal of medicine 67.5 (1979): 742-746.

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