Dehydration Types: Pathophysiology, Lab Tests and Values

By , July 31st 2013. Last reviewed 8th June 2016.

Types of Dehydration

Dehydration is water deficiency in the body. In dehydration, fluids from the blood and the space between the cells (together called extracellular space) are lost first, which is followed by loss of fluid from the cells (intracellular space). Dehydration can be categorized into isotonic, hypertonic and hypotonic, depending on how it affects the tonicity of the extracellular fluids 1.

What is tonicity?

In the human body, 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, solutions inside and outside the cells have the same ability to attract water, so they are isotonic [the Greek iso = equal]. The main solute that determines the tonicity of the extracellular fluids is sodium.

What are osmosis and osmolality?

The movement of water through the membrane from a solution with lower tonicity to a solution with higher tonicity is called osmosis. Substances that increase tonicity of solutions and thus trigger osmosis are called osmotically active substances. Sodium is the main osmotically active substance in the extracellular fluid and its amount can change significantly in dehydration. 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.

  • When dehydration does not affect the concentration of sodium in the extracellular fluid, it is called isonatremic dehydration or isotonic or iso-osmolar dehydration.
  • When dehydration results in an increased sodium concentration of the extracellular fluid, it is called hypernatremic dehydration or hypertonic or hyperosmolar dehydration.
  • When dehydration results in a decreased sodium concentration of the extracellular fluids, it is called hyponatremic or hypotonic or hypoosmolar dehydration.

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

Isotonic Dehydration

When proportionally the same amount of water and sodium is lost from the body, the sodium concentration of the extracellular fluid and hence its tonicity will not change — this is isotonic dehydration. Statistically, in most cases (~ 80%) dehydration is isotonic 2.

Lab test values in isotonic dehydration:

Blood tests:

  • Osmolality: 285-295 mOsm/kg (normal range)
  • Sodium: 130-150 mmol/liter (normal or slightly below or above 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 and not drinking enough
  • Repeated vomiting
  • Diarrhea, including secretory diarrhea in cholera 10
  • Severe bleeding
  • References: 2,3,4,10

Hypertonic (Hypernatremic) Dehydration

When proportionally more water than sodium is lost from the body, the extracellular fluid has increased concentration of sodium and becomes hypertonic regarding the intracellular fluid and therefore attracts water from the cells. This results in the cell shrinkage, which may cause brain shrinkage.

Lab test values in hypertonic dehydration:

Blood tests:

  • Osmolality: >300 mOsm/kg
  • Sodium >150 meq/liter (hypernatremia).
  • BUN (Blood Urea Nitrogen): 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 (hot weather, exercise, marathon) 20
  • Hyperventilation (prolonged fever, anxiety)
  • Poorly treated diabetes mellitus (osmotic diuresis 17, ketoacidosis 18)
  • Diabetes insipidus (both central and nephrogenic) 19
  • Heat stroke 11
  • End-stage renal failure
  • Drinking sea water in attempt to treat dehydration
  • Accidental infusion of hypertonic solutions
  • Certain diuretics
  • References: 2,3,4,6,7,11

Hypotonic (Hyponatremic) Dehydration

When proportionally more sodium than water is lost, the sodium concentration of the extracellular fluids falls, which therefore becomes hypotonic in comparison to intracellular fluid, so water moves from the extracellular fluid into the cells. This causes cell swelling, possibly resulting in 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:

  • Treating dehydration in small children or marathon runners with fluids that contain little or no sodium 8
  • Vomiting or diarrhea (in some cases) 8
  • Gastrointestinal obstruction, fistula, ileus 8
  • Heat stroke 3,4,11
  • Heat exhaustion 4
  • Pancreatitis 5,8
  • Burns 3,7,8
  • Trauma (muscle damage) 6
  • Addison’s disease 5
  • Ketonuria 8
  • Chronic malnutrition 4
  • Cystic fibrosis with excessive salt loss in sweat 9
  • Renal tubular acidosis 7
  • Salt-wasting nephropathy 5,7
  • 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
  • Accidental infusion of hypotonic fluids 4

Other references: 2,3,5,6,7,8,9,11

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

Summary of 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 replacing lost fluid by plain water without 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 in 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 Lab Test Results in Dehydration

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