A 65-year-old patient is admitted to the ICU post excision of craniopharyngioma. Explain the following blood results, showing your calculations in your answer. (15% marks)
|
Plasma Parameter |
Patient Value |
Adult Normal Range |
|
Sodium |
154* mmol/L |
135-145 |
|
Potassium |
3.6 mmol/L |
3.5-5.0 |
|
Urea |
2.7* mmol/L |
3.0-8.0 |
|
Creatinine |
37* umol/L |
45-90 |
|
Glucose |
8* mmol/L |
3.5-6.0 |
|
Urine Parameter |
Patient Value |
Adult Normal Range |
|
Sodium |
36* mmol/l |
20 |
|
Osmolality |
133 mOsm/Kg |
50-1200 |
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:
Candidates are encouraged to read the question carefully and refer to the glossary of terms to focus their answers, candidates who explained the abnormalities instead of just listing the abnormalities gave more fulsome answers and were marked accordingly. Calculations were asked for and therefore if a calculated serum osmolality was present, it was marked higher than answers without a calculation. .
A calculation was called for, but of what? Puzzling over this, the only thing one can consider calculating is the serum osmolality, which would be
= (2 × Na) + Urea + Glucose = 318.7
From this, we can then surmise that the patient has central DI, because the craniopharyngioma surgery has resulted in some damage to the posterior pituitary. Of course, one can also say this on the basis of the high serum sodium and low urine osmolality alone, but the extra step was clearly required by the examiners.