This page is dedicated to organizing various examples of standardized exam questions whose answer is diabetes insipidus. While this may seem a odd practice, it is useful to see multiple examples of how diabetes insipidus will be characterized on standardized exams (namely the boards and the shelf exams). This page is not meant to be used as a tradition question bank (as all of the answers will be the same), however seeing the classic “test” characterization for a disease is quite valuable.
KEY CHARACTERISTICS OF THIS CONDITION (ON EXAMS)
When it comes to standardized exams, each condition has its own “code” marked by key buzzwords, lab findings, clues, etc. If you are well versed in this code you will be able to more quickly identify the condition that is being discussed, and get the right answer on the exam you are taking. Below is the “code” for diabetes insipidus.
- Frequent urination: often can wake up the patients at night
- Increase thirst: patients will be drinking large volumes of water
- Head trauma/crania surgery: in cases of CENTRAL DI
- Lithium usage: in cases of NEPHROGENIC DI
- Hypernatremia: due to loss of free water
- Decreased urine specific gravity due to increased presence of water
- Increased serum osmolality: due to loss of free water
- Decreased urine osmolality: due to wasting of free water in the urine
- Water deprivation does not change urine osmolality (unlike in psychogenic polydipsia).
Response To Desmopressin:
- In CENTRAL DI administration causes a decrease in urine output, and an increase in urine osmolality
- In NEPHROGENIC DI administration causes no change to either urine output or urine osmolality.
QUESTION EXAMPLES (CENTRAL)
Question # 1
A 50 year old woman comes to the clinic because she has been experiencing frequent urination and thirst for the past week. Even though she has tried to restrict her fluid intake before bed she still awakens multiple times a night to urinate. Her medical history is notable for menopause that occurred 2 years ago. She does not take any medications. Her vital signs are within normal limits. A physical exam is non-contributory. Lab studies are shown below:
- Sodium: 148 mEq/L **
- Chloride: 112 mEq/L
- Potassium: 3.6 mEq/L
- Creatinine: 1.0 mg/dL
- Urine osmolality: 200 mOsmol/kg
- Serum osmolality: 283 mOsmol/kg
After 5 hours of a water deprivation test her urine osmolality becomes 405 mOsmol/kg and her serum osmolality becomes 304 mOsmol/kg. One hour after administration of desmopressin her urine osmolality is 1000 mOsmol/kg. Which of the following is the most likely muse of this patient’s polyuria?
Frequent urination + thirst + hypernatremia + significant increase in urine concentration with desmopressin = diabetes insipidus (central)
Question # 2
A 55-year-old man comes to the clinic complaining of increased urinary frequency for the past five weeks. He denies any burning with urination urgency or difficulty initiating urination. He explains that he wakes up several times a day to urinate. He constantly feels thirsty and he drinks water every hour to alleviate his thirst. The patient’s medications are hydrochlorthiazide and amlodipine for hypertension. His vitals are as follows: blood pressure of 120/76 mm Hg, pulse 85/min respirations 15/min. He has no fever. Lab results are as follows:
- Blood glucose: 95 mg/dL
- Serum sodium: 151 mEq/L **
- Serum potassium: 4.2 mEq/L
- Bicarbonate: 25 mEq/L
- Blood urea nitrogen: 22 mg/dL
- Serum creatinine: 1.1 mg/dL
- Serum uric acid: 10.2 mg/dL
- Serum osmolality: 315 mOsm/kg **
- Urine osmolality: 125 mOsm/kg **
What is the likely diagnosis in this patient?
Frequent urination + thirst + hypernatremia + increased serum osmolality + low urine osmolality = diabetes insipidus (assumed to be central due to lack of nephrogenic DI risk factors)
A 25 year old male comes to the clinic because he has been experiencing progressive thirst and also urinary frequency for the past 2.5 months. He explains that his thirst has become worse during this past week. He is concerned about the high volume of water he is drinking daily because it is “not normal”. A physical exam is within normal limits. A urinalysis shows a specific gravity of less then 1.006. What is the likely diagnosis in this patient?
Frequent urination + thirst + decreased urine specific gravity = diabetes insipidus (central)
A 42 year old woman comes to the clinic because she has been having issues with frequent urination. She also explains that she drinks a very large amount of water each day because she is “incredibly thirsty”. She denies taking any medications, and her past medical history is otherwise unremarkable. The patient’s urine osmolality is found to be 95 mOsm/L and serum osmolality is 302 mOsm/L. A water deprivation test is performed, and the patient’s urine osmolality remains unchanged. After the administration of desmopressin the patient’s urine osmolality is now 800 mOsm/L. What is the likely diagnosis in this patient?
Frequent urination + thirst + decreased urine osmolality + increased serum osmolality + significant increase in urine concentration with desmopressin = diabetes insipidus (central)
A 26 year old male is brought to the ER after he sustains severe head trauma when he is involved in a motorcycle accident. He is intubated and mechanically ventilated on arrival and his vital signs are stale. A physical exam reveals spontaneous roving eye movements, hyperreflexia of all 4 limbs, and the Babinski sign is present bilaterally. The patient’s intake for the past hour is 100 mL of half normal saline, and his urine output is 950 ml in this hour. Labs are collected and shown below:
- Serum sodium: 148 mEq/L ***
- Serum osmolality: 300 mOsmol/kg ***
- Urine specific gravity: 1.002 ***
What is the likely cause of this patient’s increased urinary output?
Increased urine output + hypernatremia + increased serum osmolality + low urine specific gravity + history of head trauma = diabetes insipidus (central)
A 45 year old woman undergoes the surgical removal of a large suprasellar mass. After the procedure she is comatose and is intubated and mechanically ventilated. Her vital signs are within normal limits, and her neurological exam does not reveal the presence of any focal neurological findings. Routine labs are collected and shown below:
- Serum sodium: 153 mEq/L ***
- Creatinine: 0.9 mg/dL
- Urine specific gravity: 1.003 ***
For the next day her “ins and outs” are recorded closely. She is given 2 L of IV normal saline, and her urine output is 6.5 L. What condition does this patient most likely have that would explain her increased urine output?
Increased urine output + hypernatremia + increased serum osmolality + low urine specific gravity + history of cranial surgery = diabetes insipidus (central)
QUESTION EXAMPLES (NEPHROGENIC)
A 25 year old female is admitted to the hospital because of a week long history of polydipsia and polyuria. She explains that her urge to urinate will often wake her up at night. Her past medical history is notable for a diagnosis of bipolar disorder, that has been treated with lithium for the past few years. Her dosage was increased about 7 months ago because she was experiencing recent manic episodes. The patient’s vital signs are within normal limits and a physical exam is non-contributory. The patient’s urine output is collected for the next 24 hours and totals as 7.0 L. Lab studies show a serum sodium concetration of 150 mEq/L, a serum osmolality of 320 mOsmol/kg, and a urine osmolality of 70 mOsmol/kg. After desmopressin is given, the urine output and osmolality are relatively unchanged. What is the likely diagnosis in this patient?
Frequent urination + thirst + hypernatremia + increased serum osmolality + hx of lithium usage (recent dosage increase) + no response to desmporessin = diabetes insipidus (nephrogenic)
A 35 year old woman female comes to the clinic because she has been having increased urination and thirst. She explains to the clinician that she feels like she is always thirsty, even though she is constantly drinking water. She has a past history of bipolar disorder that has been treated with lithium carbonate. The patient’s vital signs are normal, and a physical exam is non-contributory. Her serum sodium concentration is 152 mEq/L, and her urine osmolality is 150 mOsmol/kg. What is the likely diagnosis in this patient?
Frequent urination + thirst + hypernatremia + decreased urine osmolality + hx of lithium usage = diabetes insipidus (nephrogenic)
A 39 year old woman who is being treated for bipolar disorder comes to the clinic because she has been having issues with frequent urination, and excessive thirst. She explains that she has to wake up a few times each night to go and use the restroom. A urinalysis is performed 10 hours after the patient is made NPO, and it shows a urine with low specific gravity. What is the likely diagnosis in this patient?
Frequent urination + thirst + decreased urine specific gravity (in the setting of no fluid intake) + hx of lithium usage = diabetes insipidus (nephrogenic)
TESTABLE FACTS ABOUT THIS CONDITION (BEYOND ITS IDENTIFICATION)
Many questions on standardized exams go beyond simply recognizing the underlying condition. Often there are specific testable facts regarding some aspect of the disease’s pathophysiology/management/clinical implications that are commonly asked. Some of these are listed below:
- ADH stops working in the collecting ducts of the nephron
Page Updated: 01.22.2017