3. Compute for the plasma osmolality and the effective plasma osmolality. What is the importance of computing for such?

Osmolality (calc) = 2 x Na + glucose + urea **if all measurements in mmol/L Osmolality (calc) = 2 x Na + glucose/18 + urea/2.8 **if measurements are in mg/dL Given: Plasma Na = 123 mEq/L Glucose = 98 mg/dL Urea = 22 mg/dL Osmolality = 2(123) + (98/18) + (22/2.8) Osmolality = N = 275 – 295 milli-osmoles per kilogram Becker, K. (2001)Principles and Practice of Endocrinology and Metabolism 3 rd Ed.

Effective Osmolality (calc) = 2 x Na + glucose **if all measurements in mmol/L Osmolality (calc) = 2 x Na + glucose/18 **if measurements are in mg/dL Given: Plasma Na = 123 mEq/L Glucose = 98 mg/dL Urea = 22 mg/dL Osmolality = 2(123) + (98/18) Osmolality = < 275 = Hyponatremia Becker, K. (2001)Principles and Practice of Endocrinology and Metabolism 3 rd Ed.

ECF tonicity is determined primarily by the Na + concentration and patients who have hyponatremia have a decreased plasma osmolality Becker, K. (2001)Principles and Practice of Endocrinology and Metabolism 3 rd Ed.

4. What are the significance of urine osmolality (Uosm) and urine sodium (UNa)

Urine Osmolality A more exact measurement of urine concentration than specific gravity – Patient with Uosm below 100 mOsm/kg are able to appropriately suppress ADH release, leading to a maximally dilute urine – Patients with a higher urine osmolality have an impairment in water excretion due to the presence of ADH Indicated to evaluate the concentrating and diluting ability of the kidney - accurate test for decreased kidney function - monitor course of renal disease/ electrolyte therapy Rennke H., Denker, B. (2007). Renal Pathophysiology: The Essentials

Urine Sodium Helps distinguish renal from non- renal causes of hyponatremia Urine sodium exceeding 20 mEq/L is consistent with renal salt wasting – Diuretics, ACE inhibitors, mineralocorticoid deficiency, salt losing nephropathy Urine sodium less than 10 mEq/L implies avid sodium retention by the kidney – Compensation for extra-renal fluid loss (vomiting, diarrhea, sweating or third space wasting) Rennke H., Denker, B. (2007). Renal Pathophysiology: The Essentials

Effective circulating volume depletion and SIADH are the two major causes of true hyponatremia (with an inappropriately high urine osmolality) and these disordes can be distinguished by measuring the Una. – Patients with hypovolemia are sodium avid in an attempt to limit further losses Urine sodium is generally below 25 mEq/L – In comparison, patients with SIADH are normovolemic and sodium excretion is in a steady state equal to intake Urine sodium concentration is typically above 40 mEq/L Rennke H., Denker, B. (2007). Renal Pathophysiology: The Essentials