Urine Osmolality Calculator

Urine osmolality measures the concentration of dissolved particles (solutes) in urine, expressed in milliosmoles per kilogram of water (mOsm/kg). It reflects the kidney's ability to concentrate or dilute urine. CLINICAL IMPORTANCE: 1. KIDNEY FUNCTION ASSESSMENT. Measures tubular concentrating ability. More sensitive than specific gravity. Detects early kidney dysfunction. 2. FLUID BALANCE EVALUATION. Indicates hydration status. Guides IV fluid management. Monitors response to treatment. 3. DIAGNOSTIC VALUE. Diabetes insipidus (central vs nephrogenic). SIADH (Syndrome of Inappropriate ADH). Acute kidney injury classification. Distinguishing prerenal from intrinsic renal failure. NORMAL RANGES: Random urine: 300-900 mOsm/kg. After fluid restriction: >850 mOsm/kg. After water load: 40-100 mOsm/kg. Maximum concentration: ~1200 mOsm/kg. KEY CONCEPTS: Concentrated urine (>600): Kidneys conserving water. Dilute urine (<300): Kidneys excreting excess water. Isosthenuria (~300): Fixed osmolality, suggests tubular damage.

Urine osmolality can be estimated from its component solutes using a formula similar to serum osmolality calculation. THE FORMULA: Urine Osmolality = 2(Na + K) + Urea/2.8 + Glucose/18. WHERE: Na = Urine sodium (mEq/L or mmol/L). K = Urine potassium (mEq/L or mmol/L). Urea = Urine urea nitrogen (mg/dL). Glucose = Urine glucose (mg/dL). WHY MULTIPLY ELECTROLYTES BY 2: Sodium and potassium are accompanied by anions. Each cation contributes 2 osmoles (cation + anion). CONVERSION FACTORS: Urea: Divide mg/dL by 2.8 (molecular weight factor). Glucose: Divide mg/dL by 18 (molecular weight factor). SI UNITS VERSION: If urea and glucose in mmol/L, add them directly. Uosm = 2(Na + K) + Urea + Glucose. ACCURACY NOTES: Calculated value estimates measured osmolality. Direct measurement (freezing point depression) is gold standard. Gap between calculated and measured suggests unmeasured osmoles. Accuracy depends on complete solute accounting. TYPICAL CONTRIBUTIONS: Electrolytes: 40-60% of total osmolality. Urea: 30-50% of total osmolality. Glucose: Usually minimal unless glycosuria present.

High urine osmolality (>600-900 mOsm/kg) indicates concentrated urine, meaning the kidneys are conserving water. This occurs in response to various physiological and pathological conditions. PHYSIOLOGICAL CAUSES: 1. Dehydration / Fluid Restriction. Appropriate kidney response. ADH (vasopressin) is elevated. Kidneys maximize water reabsorption. Normal response: Uosm can reach 800-1200 mOsm/kg. 2. Overnight Concentration. Normal finding in morning specimen. 8+ hours without fluid intake. Expected: 500-800 mOsm/kg. PATHOLOGICAL CAUSES: 1. Volume Depletion. Blood loss, vomiting, diarrhea. Kidney appropriately conserving fluid. Uosm >500 suggests intact tubular function. 2. SIADH (Syndrome of Inappropriate ADH). ADH secreted despite low serum osmolality. Urine inappropriately concentrated (>100 mOsm/kg). Often Uosm > Serum osmolality. Key: Urine Na usually >40 mEq/L. 3. Heart Failure / Cirrhosis. Decreased effective circulating volume. Activates water retention mechanisms. Concentrated urine despite total body water excess. 4. Adrenal Insufficiency. Cortisol deficiency enhances ADH effect. Concentrated urine, hyponatremia. CLINICAL INTERPRETATION: High Uosm + High Serum Osm = Appropriate response (dehydration). High Uosm + Low Serum Osm = Inappropriate (consider SIADH). High Uosm + Normal Serum Osm = Evaluate volume status.

Low urine osmolality (<300 mOsm/kg) indicates dilute urine, meaning the kidneys are excreting excess water. This can be appropriate or pathological. PHYSIOLOGICAL CAUSES: 1. High Fluid Intake (Water Diuresis). Normal kidney response to excess water. ADH suppressed appropriately. Uosm can drop to 50-100 mOsm/kg. Resolves when intake normalized. 2. Diuretic Effect. Caffeine, alcohol inhibit ADH. Medications causing water diuresis. PATHOLOGICAL CAUSES: 1. DIABETES INSIPIDUS (DI). Central DI: Insufficient ADH production. Nephrogenic DI: Kidney doesn't respond to ADH. Characterized by: Uosm <300 despite high serum osmolality. Massive urine output (3-20 L/day). Extreme thirst (polydipsia). 2. Primary Polydipsia (Psychogenic). Compulsive water drinking. Uosm low because of excessive intake. Serum osmolality low-normal. Can cause severe hyponatremia. 3. Chronic Kidney Disease. Loss of concentrating ability. Fixed dilute urine (isosthenuria). Uosm approaches 300 mOsm/kg. 4. Hypercalcemia / Hypokalemia. Interfere with ADH action. Cause nephrogenic DI-like picture. DIAGNOSTIC APPROACH: Low Uosm + High Serum Osm = Diabetes insipidus. Low Uosm + Low Serum Osm = Water intoxication or polydipsia. Low Uosm + Normal Serum Osm = Early DI or high fluid intake. WATER DEPRIVATION TEST: Gold standard for DI diagnosis. Measures Uosm response to dehydration. Central DI: Uosm increases with desmopressin. Nephrogenic DI: Uosm doesn't respond to desmopressin.

The urine-to-serum osmolality ratio (U/S ratio or Uosm/Sosm) compares urine concentration to blood concentration, helping assess kidney function and fluid status. CALCULATION: U/S Ratio = Urine Osmolality / Serum Osmolality. INTERPRETATION: Ratio > 1: Urine more concentrated than serum. Indicates kidney is conserving water. Normal response to dehydration. Intact concentrating ability. Ratio = 1 (Isosthenuria): Urine same as plasma (~300 mOsm/kg). Indicates tubular dysfunction. Seen in chronic kidney disease. Kidney cannot concentrate or dilute. Ratio < 1: Urine more dilute than serum. Kidney excreting excess water. Or impaired concentrating ability. CLINICAL APPLICATIONS: 1. Acute Kidney Injury (AKI) Classification: Prerenal AKI: U/S ratio >1.5 (kidneys conserve water). Intrinsic renal AKI: U/S ratio ~1 (tubular damage). 2. Diabetes Insipidus: U/S ratio typically <1. Despite elevated serum osmolality. 3. SIADH: U/S ratio >1. Despite LOW serum osmolality. Key diagnostic feature. REFERENCE VALUES: Normal hydration: 1.0-3.0. Water deprivation: Can reach 3-4. Water loading: 0.2-0.5. Prerenal azotemia: >1.5. ATN: ~1.0. IMPORTANT CAVEATS: Single ratio is a snapshot in time. Serial measurements more informative. Must interpret with clinical context. Medications can affect results.

Urine osmolality is essential in diagnosing and differentiating types of diabetes insipidus (DI), a condition where the body cannot properly concentrate urine. NORMAL PHYSIOLOGY: ADH (antidiuretic hormone) released when serum osmolality rises. ADH acts on kidney collecting ducts. Water reabsorbed, urine concentrated. DIABETES INSIPIDUS TYPES: 1. CENTRAL DI. Cause: Insufficient ADH production (pituitary). Urine findings: Very dilute (<300 mOsm/kg). Often <200 mOsm/kg. Response to desmopressin (synthetic ADH): Uosm increases >50% or >600 mOsm/kg. 2. NEPHROGENIC DI. Cause: Kidney resistant to ADH. Urine findings: Very dilute (<300 mOsm/kg). Response to desmopressin: Minimal increase (<50%). Uosm remains <300 mOsm/kg. 3. PRIMARY POLYDIPSIA. Cause: Excessive water drinking. Urine findings: Dilute, but can concentrate with restriction. Serum osmolality: Low or low-normal. WATER DEPRIVATION TEST PROTOCOL: 1. Baseline serum and urine osmolality. 2. Restrict fluids (supervised). 3. Monitor hourly: weight, Uosm, Sosm. 4. Continue until: Uosm stable for 2-3 hours, OR. Weight loss >3%, OR. Sosm >295 mOsm/kg. 5. Administer desmopressin. 6. Measure Uosm 1-2 hours after. INTERPRETATION OF RESULTS: | After Dehydration | After Desmopressin | Diagnosis |. | Uosm >600 | N/A | Normal or primary polydipsia |. | Uosm <300 | Uosm >600 | Central DI |. | Uosm <300 | Uosm <300 | Nephrogenic DI |. | Uosm 300-600 | Variable | Partial DI (central or nephro) |. KEY URINE OSMOLALITY VALUES IN DI: Baseline Uosm in DI: Usually <200 mOsm/kg. Max concentrating in central DI with DDAVP: >600-800. Max concentrating in nephrogenic DI: <300.

Both urine osmolality and specific gravity measure urine concentration, but they measure different properties and have different clinical utilities. DEFINITIONS: OSMOLALITY (mOsm/kg): Number of particles per kg of water. Measures moles of solute regardless of size. Gold standard for concentration. SPECIFIC GRAVITY (no units, ~1.001-1.030): Density of urine compared to water. Depends on both number AND size of particles. Easier to measure (dipstick). KEY DIFFERENCES: 1. What They Measure. Osmolality: Particle count only. SG: Particle count + particle weight. Large molecules (protein, glucose, contrast) affect SG more than osmolality. 2. Measurement Method. Osmolality: Freezing point depression (lab). SG: Refractometer or dipstick (bedside). 3. Accuracy. Osmolality: More accurate and precise. SG: Subject to interference. CONVERSION (Approximate): Osmolality ≈ (SG - 1.000) × 35,000. Example: SG 1.020 → Osm ≈ 700 mOsm/kg. This is only approximate! WHEN THEY DIVERGE: High protein (proteinuria): SG elevated, Osm unchanged. Glucose (glycosuria): Both elevated, but SG more so. IV contrast: SG very elevated, Osm moderately elevated. Mannitol: Similar to contrast. CLINICAL IMPLICATIONS: Use osmolality when: Precise measurement needed. Suspect interfering substances. Diagnosing DI or SIADH. Research purposes. Use specific gravity when: Quick bedside assessment. Screening for dehydration. Monitoring hydration status. Resources limited. REFERENCE RANGES COMPARISON: | Osmolality | Specific Gravity | Interpretation |. | 50-100 | 1.001-1.003 | Maximally dilute |. | 300-400 | 1.010-1.012 | Isotonic |. | 600-800 | 1.020-1.025 | Concentrated |. | 1000-1200 | 1.030-1.035 | Maximally concentrated |.

Urine osmolality is crucial in the diagnostic algorithm for hyponatremia (low serum sodium), helping determine the cause and guide treatment. HYPONATREMIA DIAGNOSTIC APPROACH: Step 1: Confirm true hyponatremia (Serum Osm <280). Step 2: Assess urine osmolality. Step 3: Assess urine sodium. Step 4: Determine volume status clinically. URINE OSMOLALITY IN HYPONATREMIA: 1. UOSM <100 mOsm/kg (Maximally Dilute). Kidney is appropriately suppressing ADH. Causes: Primary polydipsia (excessive water intake). Beer potomania / tea-and-toast diet. Reset osmostat (lower set point). Key: Appropriate kidney response to low serum Osm. 2. UOSM >100 mOsm/kg (Inappropriately Concentrated). Kidney is NOT appropriately diluting urine. ADH activity present despite low serum Osm. Causes vary by volume status: HYPOVOLEMIC (Low Volume): Urine Na <20 mEq/L: GI losses, skin losses, third spacing. Urine Na >40 mEq/L: Diuretics, adrenal insufficiency, salt-wasting. Kidney conserving water appropriately for low volume. EUVOLEMIC (Normal Volume): Urine Na usually >40 mEq/L: SIADH (most common). Hypothyroidism. Adrenal insufficiency. ADH inappropriately elevated. HYPERVOLEMIC (Fluid Overloaded): Urine Na <20 mEq/L: Heart failure, cirrhosis, nephrotic syndrome. Urine Na >40 mEq/L: Renal failure. Low effective circulating volume triggers ADH. SIADH DIAGNOSIS (Key Role of Uosm): Criteria: Serum Osm <275 mOsm/kg. Urine Osm >100 mOsm/kg (usually >300). Urine Na >40 mEq/L (on normal salt intake). Clinically euvolemic. Normal thyroid and adrenal function. TREATMENT IMPLICATIONS: Uosm <100: Fluid restriction effective. Uosm >100 with SIADH: May need salt tablets or medications. Uosm >100 with volume depletion: Give saline carefully.