Case 29: Diagnostic Testing

Case 29 Index

In addition to the chemistry panel, additional labs are obtained:

all 29 labs

protein BMP

Choose the highest-yield tests that you would perform next!
Urinalysis and urine microscopy
Though this is an inexpensive, non-invasive test, in a patient with normal kidney function and hyponatremia, this is not the highest yield. Try again!
Orthostatic vital signs
Well done – orthostatics are a sensitive way to assess a patient’s volume status. This patient’s blood pressure and heart rate remain stable after changing from a sitting to standing position (and standing for 2-3 minutes), and he remains asymptomatic.
Urine protein: creatinine ratio
Though this test may be abnormal in a patient with multiple myeloma,  it is less likely to help us determine the etiology of the patient’s hyponatremia.  In a patient with normal kidney function and hyponatremia, this is not the highest yield initial test. Try again!
Urine osmolality
Excellent! Urine osmolality can help us better understand the etiology of a patient’s hyponatremia and tell us about the activity of the antidiuretic hormone (ADH). Higher osmolality suggests ADH activity is high (or ADH-dependent hyponatremia), while lower osmolality suggests lower activity of ADH (ADH-independent). This patient’s urine osmolality is 210 mOsm/kg.
Urine sodium
Excellent! Urine sodium can help us understand the effective circulating volume or effective arterial volume (EAV) along with aldosterone activity. This patient’s urine sodium is 90 meq/L.
Serum osmolality
Great! This test can help us get a better sense of our patient’s tonicity in the setting of hyponatremia. The serum osmolality is 285 mOsm/kg. (normal range: 275 – 295 mOsm/kg)
It’s time to lock in your final diagnosis. What do you think is the most likely cause of this patient’s hyponatremia?
Syndrome of inappropriate antidiuretic hormone (SIADH) due to SSRI use
Cerebral salt wasting (CSW)
Renal salt wasting (RSW)
Adrenal insufficiency
Primary polydipsia
Congestive heart failure

Click here once you’re confident in your final diagnosis!

Case 29 Index
Case 29 Introduction
Case 29 Physical Exam