MRCP2-4057
A 50 year old woman presents to her General Practitioner with a complaint of polyuria lasting for the past three months. She reports needing to urinate every 60 to 90 minutes during the day and being woken up at least twice at night. She denies any increased urinary urgency or dysuria. The patient has not made any changes to her fluid intake and consumes minimal caffeine and alcohol. She has also experienced loose stools and a weight loss of 4 Kg over recent weeks.
The patient has no significant medical history except for an appendicectomy in childhood and no previous pregnancies. Her twin sister was diagnosed with coeliac disease five years ago. She is not on any regular medications and has no known allergies. On examination, the patient appears slightly anxious and is slim. Cardiovascular, respiratory, and ocular examination is normal, and there are no skin rashes.
Initial investigations requested by the General Practitioner are as follows:
– Haemoglobin: 12.4 g/dL
– White cell count: 6.5 * 109/L
– Platelets: 329 * 109/L
– Urea: 3.5 mmol/L
– Creatinine: 75 micromol/L
– Sodium: 142 mmol/L
– Potassium: 3.7 mmol/L
– Calcium (adjusted): 2.4 mmol/L
– Haemoglobin A1C: 5.3% (reference 4-6)
– Serum glucose (random): 4.7 mmol/L
– Urine dipstick: negative for nitrites, leucocytes, glucose, ketones, protein, and beta-HCG
– Urine microscopy, culture, and sensitivities: white cell count < 10/mm³, no growth
What is the most appropriate next test to diagnose the cause of polyuria?