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Quick Reference
  • Normal Range: 1.003-1.030
  • Isosthenuria: 1.008-1.012 (fixed, suggests chronic kidney disease)
  • Low SG (Hyposthenuria): <1.010 (dilute urine)
  • High SG (Hypersthenuria): >1.025 (concentrated urine)
  • Primary Use: Assessment of hydration status and renal concentrating ability
  • Sample Type: Random urine specimen (preferably first morning void)
  • Key Point: Reflects the density of urine compared to water (1.000)

Test Description

Specific gravity (SG) measures the concentration of dissolved particles in urine relative to pure water, which has a specific gravity of 1.000. It reflects the kidney's ability to concentrate or dilute urine in response to the body's hydration status and is regulated by antidiuretic hormone (ADH/vasopressin).

Specific gravity provides a quick assessment of urine concentration and can indicate hydration status, kidney function, and certain metabolic conditions. It is measured using a refractometer, urinometer, or reagent strip during routine urinalysis.

Physiological Regulation

  • ADH (Antidiuretic Hormone): Released by the posterior pituitary in response to dehydration, causing water reabsorption in the collecting ducts and increased urine concentration (higher SG)
  • Normal Kidney Function: Healthy kidneys can vary specific gravity from 1.003 (maximum dilution) to 1.030 (maximum concentration)
  • Water Balance: SG adjusts based on fluid intake, fluid losses, and metabolic needs
Normal Ranges

Urine specific gravity normally varies throughout the day based on hydration status and kidney function. The kidneys can concentrate or dilute urine over a wide range.

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Category Specific Gravity Clinical Significance
Normal Range 1.003-1.030 Adequate kidney concentrating ability
First Morning Void 1.015-1.030 Concentrated after overnight fast
Random Specimen 1.010-1.025 Varies with hydration
Isosthenuria 1.008-1.012 Fixed SG, suggests kidney disease
Osmolality Correlation: Specific gravity correlates with urine osmolality. A specific gravity of 1.010 corresponds to approximately 350 mOsm/kg. Osmolality is more accurate for assessing true urine concentration, especially when glucose or protein is present.
Important Considerations:
  • Specific gravity measures particle density, not particle number (osmolality)
  • Large molecules (glucose, protein, contrast dye) falsely elevate SG without reflecting true renal concentrating ability
  • For accurate assessment of concentrating ability, compare SG with serum osmolality or use urine osmolality
Clinical Significance

Low Specific Gravity (Hyposthenuria, <1.010)

Low specific gravity indicates dilute urine and can result from excessive fluid intake, impaired ADH secretion or action, or intrinsic kidney disease.

Endocrine Causes:

  • Diabetes Insipidus (Central): Deficiency of ADH production by the hypothalamus or posterior pituitary
  • Diabetes Insipidus (Nephrogenic): Kidney resistance to ADH action despite adequate hormone levels
  • Psychogenic Polydipsia: Excessive water intake suppresses ADH release

Renal Causes:

  • Chronic Kidney Disease (Advanced): Loss of concentrating ability with GFR <30 mL/min
  • Acute Tubular Necrosis: Tubular damage impairs concentration mechanisms
  • Interstitial Nephritis: Medullary damage affects concentrating ability

Other Causes:

  • Diuretic Use: Loop diuretics, thiazides, osmotic diuretics increase urine volume and decrease concentration
  • Overhydration: Excessive intravenous fluids or oral intake
  • Electrolyte Disorders: Hypokalemia, hypercalcemia impair renal concentrating ability

High Specific Gravity (Hypersthenuria, >1.025)

High specific gravity indicates concentrated urine and suggests dehydration, volume depletion, or the presence of large molecules in urine.

Volume Depletion:

  • Dehydration: Inadequate fluid intake, increased insensible losses (fever, heat exposure)
  • Hypovolemia: Hemorrhage, diarrhea, vomiting, third-spacing
  • Heart Failure: Decreased renal perfusion triggers ADH release

Syndrome of Inappropriate ADH (SIADH):

  • Excessive ADH Secretion: Inappropriately concentrated urine (SG >1.020) despite low serum osmolality
  • Causes: Lung cancer, CNS disorders, medications (SSRIs, carbamazepine), post-operative state

Falsely Elevated Specific Gravity:

  • Glucosuria: Uncontrolled diabetes mellitus with glucose spillage into urine
  • Proteinuria: Nephrotic syndrome or glomerular disease with heavy protein loss
  • Radiographic Contrast: Iodinated contrast agents dramatically increase SG (can reach 1.040-1.050)

Isosthenuria (Fixed SG 1.008-1.012)

Isosthenuria refers to urine with a specific gravity fixed around 1.010, approximately equal to plasma osmolality. This indicates severe loss of kidney concentrating and diluting ability.

Clinical Significance:

  • Chronic Kidney Disease: Advanced CKD with loss of tubular function (typically GFR <30 mL/min)
  • Fixed Concentration: Kidneys cannot respond to hydration status or ADH
  • Poor Prognostic Sign: Indicates significant and often irreversible kidney damage
Interpretation Guidelines

Clinical Context

Specific gravity must always be interpreted in clinical context, considering the patient's hydration status, symptoms, and other laboratory findings.

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Finding SG Value Clinical Interpretation
Appropriate Concentration >1.020 Expected in dehydration (normal kidney response)
Appropriate Dilution <1.010 Expected with high fluid intake or IV hydration
Inappropriate Dilution <1.010 In dehydration suggests diabetes insipidus or CKD
Inappropriate Concentration >1.020 With hyponatremia suggests SIADH
Isosthenuria 1.008-1.012 Fixed SG regardless of hydration = severe CKD
Differentiating Diabetes Insipidus Types:
Water Deprivation Test:
  • Normal: SG increases to >1.020 after water deprivation
  • Central DI: SG remains low (<1.010) during deprivation, normalizes with desmopressin (ADH analog)
  • Nephrogenic DI: SG remains low (<1.010) despite water deprivation and desmopressin administration

Specific Gravity vs Osmolality

While specific gravity provides a quick estimate of urine concentration, osmolality is more accurate, especially when large molecules are present.

  • Specific Gravity: Measures particle density and weight (affected by molecule size)
  • Osmolality: Measures particle number (mOsm/kg), independent of molecule size
  • When to Use Osmolality: Suspected SIADH, diabetes insipidus evaluation, when glucose/protein/contrast present
  • SG to Osmolality Rough Conversion: Each 0.001 increase in SG above 1.000 ≈ 35 mOsm/kg
Interfering Factors

Factors That Falsely Increase Specific Gravity

  • Glucose: Uncontrolled diabetes with glucosuria (each 1 g/dL increases SG by ~0.004)
  • Protein: Heavy proteinuria in nephrotic syndrome (each 1 g/dL increases SG by ~0.003)
  • Radiographic Contrast: Iodinated contrast agents dramatically elevate SG (1.040-1.050+)
  • Medications: Mannitol, dextran, intravenous immunoglobulin (IVIG)
  • Urinary Tract Infections: Bacteria and white blood cells can slightly elevate SG

Factors That Decrease Specific Gravity

  • Diuretics: Loop diuretics (furosemide), thiazides, osmotic diuretics (mannitol)
  • Excessive Hydration: IV fluids, polydipsia
  • Alcohol: Suppresses ADH release
  • Caffeine: Mild diuretic effect
  • Chronic Kidney Disease: Impaired concentrating ability
  • Electrolyte Imbalances: Hypokalemia, hypercalcemia

Collection and Technical Issues

  • Temperature: Room temperature affects refractometer readings (calibrate to room temp)
  • Specimen Age: Bacterial growth in old specimens increases specific gravity
  • Reagent Strip Limitations: Less accurate than refractometer, limited range (1.000-1.030)
  • Alkaline Urine: pH >7 can cause reagent strip underestimation
Clinical Pearls
Clinical Pearl
"SG 1.010 = Isosthenuria": A urine specific gravity fixed at 1.010 (regardless of hydration status) indicates isosthenuria and suggests severe chronic kidney disease with loss of concentrating ability. This is approximately equal to plasma osmolality (~300 mOsm/kg).
Clinical Pearl
First Morning Void is Best: The first morning urine specimen should have the highest specific gravity (1.020-1.030) because of overnight water deprivation. If SG <1.020 on first morning void, consider impaired concentrating ability.
Clinical Pearl
Glucose and Protein Falsely Elevate SG: High specific gravity in a diabetic patient with glucosuria or a patient with proteinuria does NOT indicate adequate kidney concentrating function. Check urine osmolality for accurate assessment.
Clinical Pearl
SIADH Diagnosis Clue: In SIADH, urine is inappropriately concentrated (SG >1.020 or osmolality >100 mOsm/kg) despite low serum sodium and low serum osmolality. This is a key diagnostic finding.
Contrast Alert: After IV contrast administration for CT scans, specific gravity can be dramatically elevated (1.040-1.050). Do not interpret this as dehydration - the elevated SG is artifactual from heavy iodine molecules.
SG vs Osmolality Decision: Use specific gravity for routine screening and hydration assessment. Use osmolality when precise measurement of renal concentrating ability is needed (DI workup, SIADH diagnosis) or when glucose/protein/contrast interferes with SG interpretation.
Clinical Pearl
Dehydration Assessment: In a dehydrated patient, expect SG >1.020. If SG is low (<1.010) despite clinical dehydration, suspect diabetes insipidus or underlying kidney disease preventing adequate urine concentration.
Clinical Pearl
Loop Diuretics Confound Assessment: Patients on furosemide or other loop diuretics will have dilute urine (low SG) even if dehydrated. Discontinue diuretics before water deprivation testing if evaluating for diabetes insipidus.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape. https://emedicine.medscape.com/article/2172316-overview?form=fpf
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL. https://litfl.com/ccc-critical-care-compendium/
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
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