Laboratory findings consistent with the diagnosis of diabetes insipidus include abnormal urine osmolality, abnormal urine output, and abnormal plasma and urine ADH levels as rise in plasma/urine ADH

Measurement of the plasma sodium concentration and the urine osmolality is helpful in distinguishing between the 3 types of diabetes insipidus. Each of the three causes of polyuria – primary polydipsia, central DI, and nephrogenic DI – is associated with an increase in water output and the excretion of a relatively dilute urine. In primary polydipsia, the polyuria is an appropriate response to enhanced water intake but on the contrary the water loss is inappropriate with central and nephrogenic DI.^[1]

Some of the investigations that can be done to appropriately diagnose diabetes insipidus are:

Plasma sodium and urine osmolality:

• Plasma sodium concentration that is less than 137 meq/L associated with a low urine osmolality indicates water overload due to primary polydipsia.
• Plasma sodium concentration greater than 142 meq/L, due to water loss indicates diabetes insipidus diabetes insipidus.

Measurement of urine output:

• Clarity and usefulness of the sample collection is uncertain.

Water deprivation test:

• Water restriction or the administration of hypertonic saline (0.05 mL/kg per min for no more than two hours) can be used to differentiate central DI from primary polydipsia.

Plasma and urine ADH measurement^[2][3]

• Rise in plasma/urine ADH in response to the rising plasma osmolality levels rules out central DI.
• An appropriate rise in urine osmolality as ADH secretion is increased rules out nephrogenic DI.

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