Question 16.2

College answer

Aim: To allow the candidate to demonstrate expertise in data interpretation.

Key sources include: Common clinical practice with urinalysis, and paired urine and serum samples. CanMEDS Medical Expert.
Discussion:

This question asked for three causes of reddish urine with a clinical history of trauma and chemical exposure and half the candidates were able to do that. Correct examples included three of the following four causes: haemoglobinuria, haematuria, myoglobinuria or hydroxocobalamin. G6PD deficiency was not given marks as it is much less likely in the given scenario than the stated four causes.

Most answers were incomplete with candidates not able to describe the mechanisms or differentiation of their stated causes. This may be due to inadequate reading of the question or a knowledge deficit. The better candidate was able to address the clinical history in the stem and detail the mechanism and the differential process between the most likely diagnoses

Discussion

Ok, one does not put "explosion in a chemical warehouse" into an exam stem without expecting some kind of a reaction. The range of possibilities can be divided into "those that fit the stem" and "those that have minimal relationship with the stem but which are otherwise technically also possible". The latter would have scored minimal marks here, but are listed for the reader's amusement. Thus:

• Haematuria:
  • The patient may have had an injury to the kidneys or the renal tract (for example, as the result of being thrown by the blast)
  • This is revealed by finding red cells in the urine microscopy
• Haemoglobinuria:
  • The patient may have developed haemolysis as the result of severe thermal damage or blast injury
  • The presence of free haemoglobin in the blood, or a dipstick positive for blood in the absence of any red cells on urine microscopy, would be diagnostic
• Myoglobinuria:​​​​​​​
  • Blast injury can present with rhabdomyolysis due to widespread muscle damage
  • The resulting myoglobinuria can be detected by a urinary myoglobin level, and would also yield a a dipstick positive for blood in the absence of any red cells on urine microscopy (because the dipstick tests for haem, and myoglobin contains haem)
• Hydroxocobalamin:
  • "​​​​​​​Explosion in a chemical factory" is the college's way of making the trainees think about cyanide toxicity, which calls for a large dose of hydroxocobalamin. 
  • If for whatever reason this was not self-evident from the medication chart, one may be able to confirm the presence of a red non-haem dye in the urine by testing it for blood with a urine dipstick, which would of course be negative.

Other exciting possibilities where the explosion is irrelevant include:

• Not haematuria, but blood in urine:
  • Menstruation
  • Genital injury
• Other causes of intravascular haemolysis:
  • A mechanical valve
  • ECMO
  • G6PD deficiency
  • Sickle cell anaemia
  • Transfusion reaction
• Drugs:
  • Rifampicin, which, to be fair, is more orange
  • Isoniazid
  • Warfarin
  • Riboflavin
• Errors of metabolism
  • Porphyria
• Beetroot consumption, which causes harmless "beeturia"

Singh, Akhilesh Kumar, et al. "Differentials of abnormal urine color: a review." Ann Appl Biosci 1 (2014): R21-R25.

Wüthrich, R. P., and A. Serra. "The red urine." Therapeutische Umschau. Revue Therapeutique 63.9 (2006): 595-600.

Viswanathan, Stalin. "Urine bag as a modern day matula." International Scholarly Research Notices 2013 (2013).

Boutwell, Joseph H. "More causes of red urine." JAMA 238.14 (1977): 1501-1501.

Watts, A. R., et al. "Beeturia and the biological fate of beetroot pigments." Pharmacogenetics and Genomics 3.6 (1993): 302-311.