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.
⚠️ Disclaimer:
The content on LabTestsGuide.com is for informational and educational purposes only. We do not guarantee the accuracy, completeness, or timeliness of the information provided. Always consult qualified healthcare professionals for medical advice, diagnosis, or treatment. LabTestsGuide.com is not liable for any decisions made based on the information on this site.
