Laboratory Tests For Dehydration

Dehydration refers to the process of excessive water loss from body tissues, which is frequently accompanied by imbalances of chloride, potassium, sodium, and other types of electrolytes. This can happen anytime fluids are lost and not replaced adequately, especially when a person doesn’t drink enough fluids. There are no symptoms of early dehydration; moderate or mild dehydration may cause symptoms including headaches, dizziness, muscle cramps, fatigue, and thirst. More serious symptoms may be caused by severe dehydration, including shock, unconsciousness, low blood pressure, confusion. They might even result in death in certain cases. 

The human body is comprised of around 60% to 70% water and requires a continuous supply to function properly. Water primarily enters the body from drinking liquid and then secondarily from the food we consume. The intestines absorb the water and then carry it throughout the entire body. Water is comprised of the fluids that are contained inside of cells, within mucous membranes, inside the lymphatic system, in the spaces between tissues and cells, and the fluid part of the blood inside of our arteries and veins. Fluids may be shifted as needed from one area or compartment to another.

Most water gets filtered out of the blood and is then is reabsorbed and recirculated by the kidneys several times. Dissolved wastes and excess water create urine and then are eliminated during urination from the body. There are also additional small quantities of water that are lost continually in stool and through breathing and sweating.

The total amount of regular water loss can range from 1,500 up to 2,500 milliliters (mL) a day (around 50 to 85 ounces a day) based on these sources: 

  • Urine: an average of 1000-2000 mL per day
  • Exhaling/Evaporation: 500-1000 mL per day
  • Stool: 50 to 100 mL per day

It is a very complex process to maintain the conversation and balance of water inside the body. The kidneys belong to the feedback system that removes or conserves water by diluting or concentrating urine and through controlling sodium conservation. Sodium and other electrolytes like bicarbonate, chloride, potassium helps with regulating the balance of water at the cellular level through maintaining electrical neutrality and the acid-base balance of the body.  

This feedback system, as well as its components, is critical in helping to maintain a healthy water level inside the body. The body’s sensors perceive and respond to decreases and increases in the amounts of dissolved substances and water inside the bloodstream. As the number of dissolved particles within the blood (osmolality) increases, which increases the number of particles or decreases how much water is in the blood, the hypothalamus – a specialized gland inside the brain – secretes anti-diuretic hormone (ADH). The hormone gives the kidneys a signal to conserve water. To maintain blood volume and pressure, water moves throughout the cells into the bloodstream. If this is not corrected, the tissues of the body dry out, which can cause cells to malfunction and shrink. As levels of fluids decrease, the brain triggers a “thirst” response, which signals an individual to drink more water. These feedback systems, when they work together, are normally able to keep a dynamic fluid balance maintained.  

Dehydration takes place whenever fluids or liquids are lost at a faster rate than they are able to be replaced. That may happen with not taking in enough fluids through eating and drinking, using diuretics (medications which increase the production of urine), sweating diarrhea, or excessive vomiting. The situation may worsen if the individual loses too little sodium (hypernatremia) or too much (hyponatremia) in relation to the reduction in water. Prolonged dehydration may cause shock and result in the internal organs being damaged, especially the brain, which can lead to coma, confusion, or even potentially death.

Anybody may become dehydrated. However, the condition tends to be more serious in elderly people, the young, and individuals with weakened immune systems or underlying health conditions. Children and infants might have a hard time communicating that they are thirsty. Since they have higher body water content and faster metabolism than adults do, the fluid requirements for children are also different than those for adults. Diarrhea, vomiting, and sweating are common sources of fast fluid loss within children. Diarrheal illnesses worldwide are a very serious health threat. The World Health Organization reports that in children under five years old, the second leading cause of death is diarrheal illness. It is estimated by the U.S. Centers for Disease Control and Prevention that 2,2200 children are killed every day by diarrhea in developing countries.

Dehydration in the elderly is a very common problem. It has been associated with a range of adverse health results. Experts estimate that over 20% of elderly people who live independently within the United States are dehydrated. There have been similar dehydration rates recorded in elderly individuals living in UK residential treatment facilities.


High amounts of water may be lost very quickly with prolonged diarrhea and/or vomiting. One or both of the symptoms may be found in a wide range of conditions, including the following:

  • Irritable bowel
  • Inflammatory bowel disease
  • Malabsorption
  • Drug toxicity or overdose
  • An obstruction, such as in the digestive tract
  • Stomach flu (gastroenteritis) – inflamed digestive tract; this is a very common cause of diarrhea and vomiting in people of all ages; it might be linked to waterborne and foodborne illnesses and cause by parasitic, viral, or bacterial infections in the digestive tract

Fluids might also be lost due to the following:

  • Burns
  • Fever
  • Prolonged or intense sweating and physical exertion that might happen in athletes who are training for extended periods and/or in hot temperatures

Excessive urination might occur with:

  • Excessive bleeding
  • Using certain medications like diuretics
  • Diabetic ketoacidosis (uncontrolled diabetes)
  • Certain diseases that affect the ability of the body to conserve water and concentrate urine

A lack of fluids might also happen due to insufficient intake. That might happen:

  • With a reduced sense of appetite or thirst
  • Due to a lack of enough available water
  • In some elderly individuals who need help with accessing water
  • In individuals with sore throats or inflamed mouths who do not drink enough due to pain
  • In infants who are unable to communicate thirst

Certain rare causes include:

  • Addison Disease
  • Diabetes insipidus

Symptoms and Signs

The symptoms and signs of dehydration vary from one person to the next and on how long the reduced intake of fluids lasts.

Early dehydration does not have any symptoms. Moderate and mild dehydration might cause no or few noticeable symptoms. However, people might experience symptoms, including the following:

  • Feeling dizzy or lightheaded
  • Headache
  • Dry skin
  • Fatigue
  • An infant having mildly sunken eyes
  • Reduction in tears
  • Constipation
  • Less frequent urination and urine might be a darker yellow which indicates a concentration
  • Sticky and/or dry mouth
  • Increased thirst

Severe dehydration might cause increasingly serious symptoms and signs, including:

  • Confusions and unconsciousness in critical cases
  • Low blood pressure
  • Increased heart rate
  • Rapid breathing
  • An infant with sunken fontanelles (soft spots on top of their head)
  • An infant with sunken eyes
  • Dry skin lacking in elasticity
  • Extremely dry mucous membranes and mouth
  • Intense thirst
  • Lack of urine – no or little urine is produced, and it is darker yellow
  • Lack of sweating and tears


A dehydration diagnosis is often based on clinical symptoms and signs, and appropriate treatment is provided. Typically, laboratory testing is not required for moderate or mild dehydration. Still, various non-laboratory evaluations might be used for assessing a person with more serious symptoms and signs. 

Non-Laboratory Evaluations

They might include evaluation of:

  • State of conscious
  • If the eyes appear to be sunken, and to what degree if so?
  • Capillary refill rate – is this slower than usual? For the evaluation, the pressure gets applied to the nail bed of the patient until it turns white, which indicates the blood was forced out. The pressure is then released. It is then observed how long it takes for the nail bed to once again turn pink, which indicates the return of the blood.
  • Skin turgor – a fold of skin gets pinched and is then released. When this is done, does it just slowly relax or bounce back into shape?
  • Blood pressure – is it normal or too low?
  • Heart rate – is it normal or rapid?
  • Breathing rate – is it normal or rapid?
  • Examination of dryness of mucous membranes and skin
  • Production of tears and urine output

Laboratory Tests

In severe dehydration cases, laboratory testing is often ordered to identify acid-base and electrolyte imbalances and to evaluate general health status and kidney function. If organ dysfunction and/or imbalances are discovered, then serial testing might be conducted to monitor an individual over time as well as how they respond to treatment. Testing might include:

Basic metabolic panel (BMP) – this panel can offer information on a person’s general health in addition to kidney function and acid-base balance: Electrolytes: (bicarbonate (CO2), chloride, potassium, sodium)

Blood urea nitrogen (BUN) and creatine for evaluating kidney function. In dehydration, they are often also increased.

Urinalysis for evaluating the amount of urine production, its concentration, and color.

Complete blood count (CBC) for evaluating blood cells as well as the balance in between the liquid and solid parts of the blood; and one component specifically, the hematocrit, which dehydration may elevate.

Glucose for detecting high levels that might be an indication of uncontrolled diabetes.

Blood and/or urine osmolality – evaluate the water balance of the body.

If the dehydration cause is obvious, then it is normally not necessary to do any other testing. However, various tests might be conducted when the cause is not known, to diagnose as well as address any underlying conditions, like those that are associated with prolonged vomiting and/or diarrhea.

A stool culture to search for a bacterial infection that might be causing diarrhea.

C. diff toxin and Clostridium difficile tests.

O&P – for detecting intestinal parasites.

A wide range of other tests might be conducted depending on what the underlying cause is suspected to be of the symptoms and signs, including:

Cortisol for detecting Addison disease

Antidiuretic hormone (ADH) – this is performed rarely to help with diagnosing diabetes insipidus, or a deficiency

Liver panel – for detecting liver disease

Screening for drugs of abuse – for detecting an overdose

Blood ketones – for evaluating diabetic-ketoacidosis