Dehydration Types: Pathophysiology, Lab Tests and Values

By , July 31st 2013. Last reviewed 8th March 2018.

Types of Dehydration

Dehydration is a lack of water in the body or water deficiency. Dehydration can be categorized into isotonic, hypertonic and hypotonic, depending on how it affects the tonicity of the extracellular fluid 1. The extracellular fluid includes the blood and the fluid between the cells (interstitial fluid).

Tonicity is the ability of the solution on one side of the cell membrane to attract water from the solution on the other side of the membrane. Normally, the solutions inside and outside the cells have the same ability to attract water, so they are isotonic [from the Greek iso = equal]. The tonicity of the fluid depends on osmotically active solutes. The main osmotically active solute that determines the tonicity of the extracellular fluid is sodium.

What are osmosis and osmolality?

Osmosis is the movement of water through the membrane from a solution with lower tonicity to a solution with higher tonicity. Substances that increase the tonicity of solutions and thus osmosis are called osmotically active substances. Sodium is the main osmotically active substance in the extracellular fluid. In dehydration, the concentration of sodium in the extracellular fluid can change significantly. The measure of the amount of osmotically active substances in the solution is osmolality, which is expressed in milliosmoles of a solute per kilogram of water (mOsm/L). The normal range of the blood plasma osmolality is 285-295 mOsm/kg.

Why is it important to know if dehydration is iso-, hyper- or hypotonic? Because the type of dehydration can suggest its cause and because hypotonic dehydration must be treated with great caution to avoid severe neurological damage.

NOTE: Tonicity and osmolality are similar but not the same; for differences check here and here.

Isotonic (Isonatremic) Dehydration

Isotonic (isonatremic, iso-osmolar) dehydration occurs when proportionally the same amount of water and sodium is lost from the body, so the sodium concentration of the extracellular fluid and hence its tonicity do not change. Isotonic dehydration is the most common type of dehydration 20.

Lab test values in isotonic dehydration:

Blood tests:

  • Osmolality: 285-295 mOsm/kg (normal range)
  • Sodium: 130-150 mmol/liter (a slightly wider than normal range, which is 135-145 mmol/L)

Urine tests:

  • 24-hour urine volume: decreased
  • Urine specific gravity: increased

Possible causes of isotonic dehydration:

  • Excessive sweating
  • Repeated vomiting
  • Diarrhea, including secretory diarrhea in cholera 10
  • Severe bleeding
  • References: 3,10

Hypertonic (Hypernatremic) Dehydration

Hypertonic (hypernatremic, hyperosmolar) dehydration occurs when proportionally more water than sodium is lost from the body. This results in an increased concentration of sodium in the extracellular fluid, which becomes hypertonic regarding the intracellular fluid and therefore attracts water from the body cells. This results in the cell shrinkage, which may include a significant shrinkage of the brain cells.

Lab test values in hypertonic dehydration:

Blood tests:

  • Osmolality: >300 mOsm/kg
  • Sodium >150 mmol/liter (hypernatremia)
  • Blood Urea Nitrogen (BUN): increased

Urine tests:

  • 24-hour urine volume: decreased or increased
  • Specific gravity: increased

Possible causes of hypertonic dehydration:

  • Diarrhea in children, especially young infants (in 20% of pediatric diarrhea) 2
  • Water deprivation
  • Excessive sweating 4
  • Hyperventilation
  • Poorly treated diabetes mellitus with excessive urination (polyuria), osmotic diuresis 17 and ketoacidosis 18
  • Diabetes insipidus (both central and nephrogenic) 19
  • Heat stroke 11
  • End-stage renal failure
  • Drinking sea water
  • Accidental infusion of hypertonic solutions
  • Certain diuretics
  • References: 3,6,7,11

Hypotonic (Hyponatremic) Dehydration

Hypotonic (hyponatremic, hypoosmolar) dehydration occurs when proportionally more sodium than water is lost from the body. This results in a decreased concentration of the extracellular fluid, which becomes hypotonic in comparison to intracellular fluid, which attracts water from the extracellular fluid. This leads to cell swelling, possibly including the brain swelling (cerebral edema) 1.

Lab test values in hypotonic dehydration

Blood tests:

  • Osmolality: <250 mOsm/kg
  • Sodium <130 meq/L (hyponatremia).

Urine tests:

  • 24-hour urine: increased
  • Specific gravity: decreased
  • Sodium <20 meq/L

Possible causes of hypotonic dehydration:

  • Treating dehydration in small children or marathon runners with fluids that contain little or no sodium 8 or an accidental infusion of hypotonic fluids
  • Vomiting or diarrhea (in some cases) 8
  • Gastrointestinal obstruction, fistula, ileus 8
  • Heat exhaustion or stroke 3,11
  • Pancreatitis 5,8
  • Burns 3,7,8
  • Trauma (muscle damage) 6
  • Addison’s disease 5
  • Ketonuria 8
  • Chronic malnutrition
  • Cystic fibrosis with excessive salt loss in sweat 9
  • Salt-wasting nephropathy 5,7
  • Renal tubular acidosis 7
  • Diuretics:
    • Prolonged treatment of high blood pressure with low-sodium diet and thiazide diuretics ( hydrochlorothiazide) 5,7,8.
    • Furosemide 5,7
    • Osmotic diuretics (mannitol) 5,7,8
  • Other references: 3,5,6,7,8,11

NOTE: certain conditions, such as diarrhea, can cause isotonic, hypertonic or hypotonic dehydration.

Possible Electrolyte Imbalances in Dehydration

  • Dehydration can be present without any significant electrolyte imbalance.
  • Hypernatremia is a result of dehydration.
  • Hyponatremia is not a result of dehydration but a result of treatment of dehydration with fluids that do not contain enough sodium.
  • Hyperkalemia may be triggered by dehydration; it can be also present in acute or chronic kidney failure, Addison’s disease and diabetes 1, or destruction of the red blood cells in burns 18,19.
  • Hypokalemia can develop due to vomiting or diarrhea 16.
  • Mild and transient hypermagnesemia 13 and hypercalcemia 14 may result from decreased amount of water in the blood (pseudo-hypermagnesemia and pseudo-hypercalcemia).
  • Hypocalcemia and hyperphosphatemia can occur in severe diarrhea 15.

Other Possible Lab Test Results in Dehydration

  • Elevated liver enzymes (AST, ALT, LDH) and pancreatic enzymes (lipase, amylase) 12
  • Decreased glomerular filtration rate (GFR) 12