A 32 year old man is referred to you for a complain of polyuria and polydipsia. He was well until 2 weeks ago when the symptoms started. He reports drinking at least 5 litres of water per day and frequently going to the washroom to urinate. He sleeps with a 2 litre water bottle at his bedside and wakes up 2-3 times per night to urinate and drink water to quench his thirst.

He has not noticed any other symptoms. He has no past medical history. He does not take any medications. There’s family history of type 2 diabetes in his father. He does not smoke or drink alcohol. He works as a construction worker.

Labs done during the first visit to his family physicians office:

Test	Value
Na	143 mmol/L
Plasma osmolality	292 mosmol/kg
Urine osmolality	200 mosmol/kg
Glucose	6.1 mmol/L

What is your differential diagnosis? Select all that apply.

Syndrome of inappropriate ADH secretion.
Partial central diabetes insipidus.
Complete central diabetes insipidus.
Partial nephrogenic diabetes insipidus.
Complete nephrogenic diabetes insipidus.
Cerebral salt wasting syndrome.
Primary polydipsia.

You decide to perform a water deprivation test, which shows the following:

Time (min)	Na (mmol/L)	Plasma osm (mosmol/kg)	Urine output (mL)	Urine osm (mosmol/kg)	Intervention
0	140	282	150	100
60	143	285	110	130
120	146	297	170	220	ddAVP 4 µg administered IV
150	146	297	60	540

What is your diagnosis? Select one answer.

Syndrome of inappropriate ADH secretion.
Partial central diabetes insipidus.
Complete central diabetes insipidus.
Partial nephrogenic diabetes insipidus.
Complete nephrogenic diabetes insipidus.
Cerebral salt wasting syndrome.
Primary polydipsia.

Answer

Labs done during the first visit to his family physicians office:

Test	Value
Na	143 mmol/L
Plasma osmolality	292 mosmol/kg
Urine osmolality	200 mosmol/kg
Glucose	6.1 mmol/L

What is your differential diagnosis? Select all that apply.

~~Syndrome of inappropriate ADH secretion.~~ — presents with hyponatremia, low plasma osmolality, high urine osmolality
Partial central diabetes insipidus.
Complete central diabetes insipidus.
Partial nephrogenic diabetes insipidus.
Complete nephrogenic diabetes insipidus.
~~Cerebral salt wasting syndrome.~~ — is associated with hyponatremia
~~Primary polydipsia.~~ — is associated with low normal serum sodium, typically < 137

You decide to perform a water deprivation test, which shows the following:

Time (min)	Na (mmol/L)	Plasma osm (mosmol/kg)	Urine output (mL)	Urine osm (mosmol/kg)	Intervention
0	140	282	150	180
60	143	285	110	220
120	146	297	170	275	ddAVP 4 units administered IV
150	146	297	60	400

What is your diagnosis? Select one answer.

Syndrome of inappropriate ADH secretion.
Partial central diabetes insipidus.
Complete central diabetes insipidus.
Partial nephrogenic diabetes insipidus.
Complete nephrogenic diabetes insipidus.
Cerebral salt wasting syndrome.
Primary polydipsia.

Interpretation of water deprivation test

The water deprivation test shows elevation in serum sodium and osmolality without an adequate rise in urine osmolality, up to 120 min. This is consistent with diabetes inspidus, which could either be central or nephrogenic.
Once plasma osmoality reaches 295-300 mosmol/kg or greater, ddAVP is administered to differentiate between central (responds) and nephrogenic (does not respond).
A rise of urine osmolality by 100% after ddAVP indicates indicates complete DI. This was not the case.
Lack of change to urine osmolality indicates complete nephrogenic DI. This was not the case, eihter.
A rise of u.osm by > 45% (but < 100%) indicates partial central DI.
A rise of u.osm by < 45% may be due to either partial nephrogenic or partial central DI. However, the absolute u.osm after ddAVP is typically > 300 mosmol/kg in partial central DI (as is the case here) but remains low, under plasma osm, in partial nephrgenic DI (which was not the case).
Therefore, the diagnosis is partial central DI.