Urine Specific Gravity Estimation Calculator: Assessing Renal Concentration Capacity:
Urine specific gravity calculator: Estimate renal concentration ability using urine osmolality or electrolyte values. Formula, interpretation, and clinical applications for dehydration, SIADH, and AKI.
Primary Method (Osmolality-Based):
USG =
[ Urine Osmolality (mOsm/kg) / 43 ] × 0.001 + 1.000
Alternative Method (by solutes):
Estimated SG =
1.000 + (Ureamg/dL × 0.0003) + (Glucosemg/dL × 0.0004) + (Proteinmg/dL × 0.0003)
Alternative Method (Electrolytes-Based):
USG ≈
1.000 +
2×(
Na⁺ + K⁺
) +
Urea
2.8
+
Glucose
18
40,000
Key Components:
• Osmolality Method: Gold standard for USG calculation
• 43 mOsm/kg: Conversion constant
• Alternative Method: For when osmolality unavailable
• Normal Range: 1.003-1.030
• Critical Values: <1.002 or >1.035
• 43 mOsm/kg: Conversion constant
• Alternative Method: For when osmolality unavailable
• Normal Range: 1.003-1.030
• Critical Values: <1.002 or >1.035
Methodology:
• Osmolality: Measured by freezing point depression
• Electrolytes: Standard urinalysis measurements
• Urea/Glucose/Protein: Must be in mg/dL units
• Primary method preferred when possible
• Alternative method for screening
• Electrolytes: Standard urinalysis measurements
• Urea/Glucose/Protein: Must be in mg/dL units
• Primary method preferred when possible
• Alternative method for screening
Calculation Example:
• Osmolality Method: 650 mOsm/kg
USG = (650/43)×0.001 + 1.000 = 1.015
• Alternative Method: Urea 450, Glucose 120, Protein 30
SG = 1.000 + (450×0.0003) + (120×0.0004) + (30×0.0003) = 1.000 + 0.135 + 0.048 + 0.009 = 1.192
USG = (650/43)×0.001 + 1.000 = 1.015
• Alternative Method: Urea 450, Glucose 120, Protein 30
SG = 1.000 + (450×0.0003) + (120×0.0004) + (30×0.0003) = 1.000 + 0.135 + 0.048 + 0.009 = 1.192
Clinical Applications:
• Assess hydration status
• Evaluate renal concentrating ability
• Monitor fluid therapy effectiveness
• Screen for SIADH vs. psychogenic polydipsia
• Detect false negatives in urine drug screens
• Evaluate renal concentrating ability
• Monitor fluid therapy effectiveness
• Screen for SIADH vs. psychogenic polydipsia
• Detect false negatives in urine drug screens
Sampling Protocol:
• First morning specimen preferred
• Clean-catch midstream collection
• Process within 2 hours of collection
• Refrigerate if delayed analysis
• Avoid contamination
• Note recent IV contrast/medications
• Clean-catch midstream collection
• Process within 2 hours of collection
• Refrigerate if delayed analysis
• Avoid contamination
• Note recent IV contrast/medications
Limitations:
• Alternative method less accurate with high glucose/protein
• Radiocontrast agents elevate osmolality
• Affected by temperature (refractometer)
• Glycosuria causes overestimation
• Not reliable in severe renal impairment
• pH extremes affect reagent strip readings
• Radiocontrast agents elevate osmolality
• Affected by temperature (refractometer)
• Glycosuria causes overestimation
• Not reliable in severe renal impairment
• pH extremes affect reagent strip readings
Critical Clinical Notes:
• Osmolality method more accurate for hypertonic specimens
• Values >1.035 suggest radiocontrast/foreign substances
• Alternativemethod underestimates in ketonuria
• Always correlate with serum osmolality
• Consider diabetes insipidus if USG <1.005 with high serum osmolality
• Osmolality method more accurate for hypertonic specimens
• Values >1.035 suggest radiocontrast/foreign substances
• Alternativemethod underestimates in ketonuria
• Always correlate with serum osmolality
• Consider diabetes insipidus if USG <1.005 with high serum osmolality
🧪 Urine Specific Gravity Estimation Calculator
🔍 Overview:
Urine Specific Gravity (USG) reflects urine concentration and helps assess hydration, kidney function, and metabolic disorders.
📀 Formula:
- USG (by osmolality) = (Urine Osmolality ÷ 43) × 0.001 + 1.000
- USG (by solutes) = 1.000 + (Urea × 0.0003) + (Glucose × 0.0004) + (Protein × 0.0003)
- USG (by electrolytes) = (1.000 + [2 × (Na⁺ + K⁺) + Urea ÷ 2.8 + Glucose ÷ 18]) ÷ 40,000
🖊️ Enter the Following Values:
Core Formulas for Estimation:
1. Primary Method (Osmolality-Based):
Primary Method (Osmolality-Based):
USG =
[ Urine Osmolality (mOsm/kg) / 43 ] × 0.001 + 1.000
Example:
- Urine Osm = 600 mOsm/kg
- SG = (600 / 43) x 0.001 + 1.000
- SG = (13.95) x 0.001 + 1.000
- SG = 0.014 + 1.000 = 1.014
2. Alternative Method:
Alternative Method:
Estimated SG =
1.000 + (Ureamg/dL × 0.0003) + (Glucosemg/dL × 0.0004) + (Proteinmg/dL × 0.0003)
Example:
- Urine Urea: 150 mg/dl
- Urine Glucose: 100 mg/dl
- Urine Protein: 30 mg/dl
Calculation
- SG = 1.000 + (Urea× 0.0003) + (Glucose× 0.0004) + (Protein× 0.0003)
- SG = 1.000 + (150 × 0.0003) + (10 × 0.0004) + (30 × 0.0003)
- SG = 1.000 + (0.045) + (0.04) + (0.009) = 1.094
2. Alternative Method (Electrolytes Based):
USG ≈
1.000 +
2×(
Na⁺ + K⁺
) +
Urea
2.8
+
Glucose
18
40,000
Note: Urea and glucose must be in mg/dL units
• Na⁺ and K⁺ in mmol/L
Example:
- Urine Urea: 150 mg/dl
- Urine Glucose: 100 mg/dl
- Urine Sodium: 140 mEq/L
- Urine Potassium: 20 mEq/L
Calculation
- SG = 1.000 + [2×(Na⁺ + K⁺) + Urea/2.8 + Glucose/18] / 40,000
- SG = 1.000 + [2×(140 + 20) + 150/2.8 + 100/18] / 40,000
- SG = 1.000 + [2×(160) + 53.57 + 5.56] / 40,000
- SG = 1.000 + [320 + 53.57 + 5.56] / 40,000
- SG = 1.000 + 379.13 / 40,000
- SG = 1.000 + 0.0095 = 1.009
Clinical Interpretation
| Specific Gravity | Clinical Significance | Common Conditions |
|---|---|---|
| 1.001–1.010 | Hypotonic Urine | Diabetes Insipidus, Polydipsia |
| 1.010–1.015 | Isosthenuria | ATN, CKD |
| 1.020–1.030 | Normal Concentration | Euvolemia |
| > 1.030 | Hypertonic Urine | Dehydration, SIADH, Glycosuria |
Conversion Guide
| Substance | Conversion to mmol/L | Example |
|---|---|---|
| Urea | mg/dL ÷ 2.8 | 140 mg/dL → 50 mmol/L |
| Glucose | mg/dL ÷ 18 | 90 mg/dL → 5 mmol/L |
| Protein | mg/dL ÷ 100* | 500 mg/dL → 5 g/L |
*Add 0.001 to SG per 3.3 g/L protein (e.g., 5 g/L → +0.0015)*
When to Use This Calculator
- Dehydration Assessment:
- SG > 1.025 + High Serum Osm → Volume Depletion
- Polyuria Workup:
- SG < 1.010 + High Serum Osm → Diabetes Insipidus
- Hyponatremia:
- SG > 1.020 → SIADH
- SG < 1.010 → Primary Polydipsia
- AKI Differentiation:
- SG > 1.020 → Prerenal AKI
- SG ≈ 1.010 → Intrinsic Renal AKI
Limitations & Adjustments
| Interfering Substance | Effect on SG | Correction |
|---|---|---|
| Glucose (per 1 g/dL) | ↑ 0.004 | Subtract 0.004 per 1 g/dL |
| Protein (per 1 g/dL) | ↑ 0.003 | Subtract 0.003 per 1 g/dL |
| Radiocontrast | Falsely ↑ | Use osmolality instead |
| Ketonuria | Mild ↑ | Clinically insignificant |
Comparison to Measured Specific Gravity
| Method | Accuracy | Best For |
|---|---|---|
| Refractometer | High (Gold Standard) | Most clinical settings |
| Reagent Strips | Moderate | Rapid bedside screening |
| Osmolality Formula | Moderate-High | When refractometer unavailable |
| Electrolyte Formula | Low-Moderate | Initial estimation only |
Clinical Pearls
- Critical Values:
- SG < 1.005 → Complete DI
- SG > 1.035 → Severe dehydration or osmotic load
- Diuretic Effects:
- Loop diuretics → Isotonic urine (SG 1.010) despite dehydration
- Pediatric Note:
- Neonates normally have SG 1.012 (immature concentration ability)
- AKI Red Flag:
- SG > 1.020 + FENa <1% → Prerenal Azotemia
⚠️ Pitfall Alert: SG >1.035 often indicates radiocontrast or paraproteins – not true dehydration!
🔬 Pro Tip: For hyponatremia, always pair SG with urine sodium and serum osmolality.
