Differentials
Common
Central diabetes insipidus
History
polyuria; history of trauma, neoplasm, or neurosurgery; history of phenytoin and/or ethanol ingestion
Exam
may have signs of head trauma, pituitary surgery, and/or severe hypernatraemia (hyperthermia, delirium, seizures, and coma)
1st investigation
- serum sodium:
may be elevated
- serum osmolality:
normal or elevated
- urine osmolality:
<300 mmol/kg (300 mOsm/kg)
More - 24 hour urine collection:
>50 ml/kg/24 hours
Other investigations
- water deprivation test:
urine osmolality < serum osmolality following dehydration
More - AVP (desmopressin) stimulation test:
>50% increase in urine osmolality following desmopressin
More - MRI brain:
may show pituitary or parapituitary mass, congenital abnormalities, abnormal pituitary stalk; evidence of trauma or previous surgery
More
Nephrogenic diabetes insipidus
History
family history of nephrogenic diabetes insipidus; history of underlying kidney disorder (e.g., sickle cell disease, polycystic disease, and obstructive uropathy); lithium, colchicine, gentamicin, rifampin (rifampicin), or propoxyphene (dextropropoxyphene) use; patient may be pregnant in second or third trimester[14]
Exam
signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, and hypotension
1st investigation
Hyperosmolar hyperglycaemic state (HHS)
History
older patient with known type 2 diabetes mellitus, weight loss, lethargy, visual disturbances, altered mental status common
Exam
signs of severe volume depletion (dry mucous membranes, poor skin turgor, drowsiness), blurred vision; stupor/coma[12]
1st investigation
Post-obstructive diuresis
History
most commonly after relief of bilateral obstruction or obstruction of a solitary kidney, especially after relief of urinary retention (e.g., as may occur with benign prostatic hypertrophy, prostatitis, prostate cancer, nephrolithiasis, bladder tumours, urethral strictures)
Exam
signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
- none:
diagnosis is clinical
Other investigations
Severe diarrhoea
History
passage of frequent loose stools, may be colicky abdominal pain, dizziness, weight loss, lethargy, thirst; hx of tropical sprue, pancreatitis, lactose intolerance, or bowel surgery may be present (suggesting carbohydrate malabsorption); may be history of contact with infected people, food, or fluids suggesting viral gastroenteritis
Exam
abdominal discomfort; signs of volume depletion: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
Vomiting
History
abdominal pain; may be a history of binge eating and vomiting consistent with a diagnosis of bulimia
Exam
abdominal discomfort; signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, and hypotension
1st investigation
- basic metabolic panel:
may show decreased potassium and chloride
More
Limited access to water
History
may affect infants, disabled people, people with impaired mental status, postoperative patients, intubated patients
Exam
signs of volume depletion: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
- none:
diagnosis is clinical but supportive tests necessary to exclude associated electrolyte abnormalities
Other investigations
- metabolic panel:
may show hypokalaemia and/or renal impairment
Uncommon
Primary hypodipsia
History
lack of thirst, history of hypothalamic brain tumour, vascular occlusion in the brain, or granulomatous disease (e.g., sarcoidosis)
Exam
signs of volume depletion: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output; signs of underlying abnormality may be present
1st investigation
- serum sodium:
elevated
More
Other investigations
- serum urea and creatinine:
elevated
- serum osmolality:
markedly elevated
More - urine osmolality:
markedly elevated
Cushing syndrome
History
weight gain, skin changes (purple stretch marks, easily bruised, other signs of skin thinning); difficulty climbing stairs, getting out of a low chair, and raising arms; menstrual irregularities, amenorrhoea, infertility, and decreased libido in women; men may be impotent or have decreased libido; depression, cognitive dysfunction, and emotional lability; poor wound healing; headaches; polyuria and nocturia; visual problems, galactorrhoea
Exam
moon face, facial plethora, supraclavicular and/or dorsicocervical fat pads, truncal obesity, purple striae, proximal muscle weakness, hirsutism, growth retardation (children), hypertension; signs of hypervolaemia: irritating cough, dyspnoea, jugular vein distention, and crepitations on auscultation
1st investigation
Other investigations
- serum glucose:
may be elevated
More
Primary aldosteronism
History
family history of primary aldosteronism or early onset hypertension and/or stroke; age ≥20 years; nocturia, polyuria, muscle weakness, paraesthesias, muscle cramps, palpitations, lethargy, mood disturbance (irritability, anxiety, depression), difficulty concentrating
Exam
hypertension, proximal muscle weakness may be present; signs of hypervolaemia: irritating cough, dyspnoea, jugular vein distention, and crepitations on auscultation
1st investigation
- serum sodium:
normal or elevated
More - serum potassium:
normal or decreased
More - plasma aldosterone concentration (PAC):
high
More - plasma renin activity (PRA):
low (usually <1 nanogram/mL/hour)
More - aldosterone-to-renin ratio (ARR):
increased: ratio is alternatively >55 for aldosterone in picomol/L and direct active renin concentration in mU/L; ratio >20 for plasma aldosterone in nanograms/100 decilitre and plasma renin activity in nanograms/mL/hour
More
Other investigations
Laxative and bowel cleansing agent use
History
laxative use such as lactulose, glycerin, or sorbitol (may be from charcoal-sorbitol in drug-overdose treatment, sorbitol as a sugar replacement in diet foods/drinks and diet aids); loose stools
Exam
abdominal distension and tenderness
1st investigation
- none:
diagnosis is clinical but supportive tests necessary to exclude associated electrolyte abnormalities
Other investigations
- serum potassium:
hypokalaemia may be present
- urine osmolality:
hypertonic urine
More
Enteric fistulae
History
history of Crohn's disease; recurrent urinary tract infections and pneumaturia (enterovesical fistulae), passage of gas or faeces through the vagina (enterovaginal fistulae), bowel contents draining to the surface of the skin (enterocutaneous fistulae), feculent vomiting (cologastric fistulae)
Exam
presence of bubbles in the urine (pneumaturia); passage of faeces through the vagina; bowel contents draining to surface of skin; feculent vomiting
1st investigation
- none:
diagnosis is clinical
Other investigations
Diuretics
History
loop diuretic (e.g., furosemide and torsemide) or osmotic diuretic (e.g., intravenous mannitol) use, especially in older or intubated patients; may have hypertension or pulmonary, peripheral, or cerebral oedema
Exam
signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
- none:
diagnosis is clinical but additional tests may be indicated to exclude associated abnormalities or support diagnosis
Other investigations
- serum potassium:
hypokalaemia may be present
- urine osmolality:
isotonic urine
More
Heat exposure
History
exposure to high environmental temperatures
Exam
evidence of profuse sweating; signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, and hypotension
1st investigation
- none:
diagnosis is clinical but supportive tests necessary to exclude associated electrolyte abnormalities
Other investigations
- serum potassium:
hypokalaemia may be present
- urine osmolality:
hypertonic urine
More
Exercise
History
recent exercise
Exam
evidence of profuse sweating
1st investigation
- none:
diagnosis is clinical but supportive tests necessary to exclude associated electrolyte abnormalities
Other investigations
- basic metabolic panel:
hypokalaemia may be present
- urine osmolality:
hypertonic urine
More
Fever
History
febrile illness
Exam
temperature ≥38°C (≥100.4°F); additional signs depend on related illness
1st investigation
- none:
diagnosis is clinical and further tests usually determined by underlying illness
Other investigations
Severe burns
History
history of burns
Exam
extensive skin loss and ulcerations
1st investigation
- none:
diagnosis is clinical but supportive tests necessary to exclude associated abnormalities
Inadequate breastfeeding of infants
History
history of inadequate breastfeeding without supplementation in an infant
Exam
signs of volume depletion: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, and hypotension; signs of severe hypernatraemia may be present: hyperthermia, delirium, seizures, and coma
1st investigation
- none:
diagnosis is clinical but additional tests may be indicated to exclude associated abnormalities
Other investigations
- serum potassium:
hypokalaemia may be present
- ABG:
metabolic acidosis
Salt ingestion
History
history of replacing sugar with salt in infant formula (accidental or intentional), history of ingesting highly concentrated emetic agent or gargle (e.g., Epsom salts)
Exam
signs of hypervolaemia: peripheral oedema, hypertension, irritating cough, dyspnoea, jugular vein distention, and crepitations on auscultation
1st investigation
- serum sodium:
elevated, may be >170 mmol/L (170 mEq/L)
More
Other investigations
High-protein diet
History
history of high-protein diet or high-protein tube feeds
Exam
signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
- none:
diagnosis is clinical
Other investigations
Use of intravenous sodium chloride
History
current hospital admission (especially in the intensive care unit); more commonly history of hypertonic sodium chloride administration (e.g., for irrigation of hydatid cysts), isotonic sodium chloride administration in a patient who has diabetic ketoacidosis (DKA) with an osmotic diuresis may also result in hypernatraemia[18]
Exam
signs of hypervolaemia may be present: weight gain, hypertension, irritating cough, dyspnoea, jugular vein distention, and crepitations on auscultation
1st investigation
- serum sodium:
elevated, may be >170 mmol/L (170 mEq/L)
More
Other investigations
Use of intravenous sodium bicarbonate
History
current hospital admission (especially in the intensive care unit), history of severe metabolic acidosis
Exam
signs of hypervolaemia may be present: weight gain, hypertension, irritating cough, dyspnoea, jugular vein distention, and crepitations on auscultation
1st investigation
- serum sodium:
elevated, may be >170 mmol/L (170 mEq/L)
More
Other investigations
Enteral feeding
History
high-protein diet, limited access to water
Exam
signs of volume depletion may be present: altered mental status, poor skin turgor, dry mucous membranes, sunken eyes, irritability, hypotension, decreased urine output
1st investigation
- none:
diagnosis is clinical
Other investigations
Use of this content is subject to our disclaimer