Case 47: Diagnostic Testing

Case 47 Index

The patient’s admission labs were all within normal limits. Here are her labs today:

Case 47 BMP

Over the past several days, the team noticed her serum sodium level rise from 140 mEq/L (normal) to 162 mEq/L today.

Basic Lab Ref

What is this patient’s Free Water Deficit (FWD), if she weighs 60 kilograms?

Free Water Deficit

What are the most important next steps in evaluation of this patient’s hypernatremia (multiple are correct)?
Intake / Output
It is very important to quantify the patient’s input (particularly intravenous fluids and their sodium concentrations) and output. In the past 24 hours, the patient received 0.5 L of normal saline and had minimal oral intake. Her urine output is approximately 200ml/hour (4.8L in past 24 hours), and she had 1 loose bowel movement.
Urine osmolality
That’s right! Urine osmolality is crucial to assess the patient’s excretion of free water. Her urine osmolality is 130 mosm/kg
Water deprivation test
Given this patient’s urine osmolality, this test is a great choice. After this test, the repeat urine osmolality is 130 mosm/kg.
Magnetic resonance imagine (MRI) of the brain
While this test may be helpful to evaluate certain etiologies of hypernatremia, there are a few higher yield tests. Try again!
Which of the following is the most likely primary etiology of this patient’s hypernatremia?
Central diabetes insipidus
Nephrogenic diabetes insipidus
Gastrointestinal losses
Insensible & sweat losses
Osmotic diuresis
Primary polydipsia

Based on this patient’s clinical history and presentation, what would expect to be her response to administration of desmopressin/exogenous antidiuretic hormone (ADH)?
No change in urine osmolality
Correct! Based on the most likely disorder, the administration of exogenous ADH would be unlikely to have a significant effect on urine osmolality.
Rise in urine osmolality
If the primary issue here were a lack of ADH, we would expect the urine osmolality to increase with exogenous ADH administration. However, the issue here is more likely localized to the kidneys, therefore we would not expect a significant response.

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Case 47 Index
Case 47 Introduction
Case 47 Physical Exam