Diabetes Insipidus, Part 2

Diagnosing DI

Polyuria = urine vol > 3L in 24 hrs - there are many causes of polyuria and it is important to figure out if the cause is DI or something else prior to establishing treatment

Urine osmolality (osm) of > 300 mOsmol/kg + high serum glucose --> think diabetes mellitus
Urine osmolality (osm) of > 300 mOsmol/kg + high serum urea --> think renal dz
Urine osmolality (osm) of < 200 mOsmol/kg + polyuria --> think DI

So you have a patient that has urine ohm < 200 + polyuria and you are thinking DI... how do you differentiate between central DI and nephrogenic DI?

Answer: water deprivation test

Findings:
Central DI
urine osm < plasma osm after dehydration
after ADH injections urine osm increases by >50%

Psychogenic DI

urine osm > plasma osm after dehydration

after ADH injections urine osm increases minimally

Nephrogenic DI

urine osm < plasma osm after dehydration

after ADH injections urine osm increases by <50%

Source: 
Makaryus/Mcfarlane. DI: diagnosis and treatement of a complex disease Cleveland Clinic Journal of Medicine Jan 2006 Vol 73:1
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