Polyuria Flashcards
What is polyuria vs high urinary frequency
What is diagnositc of polyuria
Polyuria= production of abnormally large volumes of dilute urine
In both polyuria and high urinary frequency the patient will be passing urine more often than before.
> 3L urine in 24hr sample= polyuria
How does chronic renal failure cause polyuria? What other cause leads to polyuria by the same mechanism
Chronic renal failure and hypercalcaemia (e.g. due to bone metastases) cause
polyuria by inducing nephrogenic diabetes insipidus.
Why might steroids lead to polyuria
steroids and Cushing’s syndrome can cause polyuria by causing diabetes mellitus, but this is rare.
Why do you ask about nocturia
During the night, the kidneys concentrate the urine to retain fluid during sleep (fluid intake is zero at this point)
Nocturia (in the absence of other causes like BPH) is often one of the earliest signs of a loss of concentrating
ability.
Makes primary polydipsia less likely
T/F a UTI could cause polydipsia
No. UTI would cause increased frequency but not polyuria.
However, UTIs could point you towards something that does cause polyuria (diabetes)
T/F a UTI could cause polyuria
No. UTI would cause increased frequency but not polyuria.
However, UTIs could point you towards something that does cause polyuria (diabetes)
Why could cancer cause polyuria
In older patients, ask about cancer and known bony
involvement, as this is one of the most common causes of hypercalcaemia.
Hypercalcaemia can induce nephrogenic diabetes insipidus
T/F Primary polydipsia is more common in patients with a history of psychiatric
disorders
T
Investigations
Bloods: -WCC for infection -HbA1c/fasting glucose -Calcium -U&Es -eGFR Urine dipstick test
Urine 24hr collection test
Desmopressin test. Stop them from drinking water. Measure their osmolality and levels of VP. The osmolality will probably be high and the VP either high or low. After giving them desmopressin, measure again. If the osmolality reduces, cranial.
What can the urine osomolality tell you about the mechanism of polyuria
- High urine osmolality. This occurs when solutes like glucose, Na+ are not reabsorbed into the plasma. They remain in the urine and keep water with them. Leading to high urine osmolality and volume (>300mOsm)
- Low urine osmolality. This happens when there is pathology in the distal tubule or deficient ADH pathway. Low urine osmolality. (<250mOsm)
Why would you look at serum sodium/electrolytes in the context of polyuria
In primary polydipsia, the serum sodium will be dilute due to the excessive water intake.
In all other pathologies, the serum sodium will be
concentrated as the patient is dehydrated. These abnormalities will be mirrored in the other electrolytes.
You will also need the potassium if you want to
calculate serum osmolarity.
What should you do before giving a water deprivation test
Th is is a very labour-intensive test, so before you start, you should confi rm there is true polyuria by ensuring a 24-hour urine collection is >3 L.
What is the danger with the water deprivation test
Th e test is also potentially dangerous in patients with diabetes insipidus as the deprivation of water intake can cause hypovolaemia, so patients must be weighed throughout and the test must be stopped if there is body weight loss of >3%.
What is the WHO criteria for diabetes mellitus
i. Fasting plasma glucose <7mM
ii. Plasma glucose >11.1mM 2hrs after ingesting 75g of glucose (oral glucose tolerance test)
Criteria must be met on 2 separate occasions or on one occasion with the presence of symptoms suggesting DM
Reasons for going into DKA
HAPPENS IN TYPE I DIABETICS (not type II, because even the smallest amount of insulin would switch of ketosis)
Undiagnosed T1DM, non-compliance or illness
