Lab Values That Indicate Dehydration: A Guide for Practitioners and Patients

Water is essential to human survival and wellness, playing critical roles in many bodily processes. When inadequate water intake and dehydration occur, various health consequences and potential dysfunction can result. Dehydration can affect all individuals but is especially concerning in vulnerable populations such as children and the elderly.

This article explores the lab values and tests that may indicate dehydration and covers common causes, symptoms, and conditions that can lead to dehydrated states. Accurate diagnosis and prompt management are crucial to help prevent serious complications. Understanding these lab values is vital to managing dehydration effectively.

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Dehydration: Causes and Symptoms

Dehydration occurs when the total amount of water excreted from the body exceeds the total water intake from foods and ingested fluids. Water losses occur via the kidneys as urine, through the skin as sweat, by the lungs during exhalation, and through the gastrointestinal tract as stool or vomit. 

Dehydration is relatively common, impacting up to 28% of the adult population, and can significantly impact human health, potentially leading to adverse outcomes if not addressed quickly. 

Common Causes of Dehydration

There are many reasons why individuals may become dehydrated, the most common of which include: 

• Excessive sweating from heat, exercise, or fever
• Insufficient water intake through diet and fluids
• Medications such as diuretics that cause increased urination
• Vomiting/diarrhea 
• Excessive laxative use

Symptoms of Dehydration

The presenting symptoms of mild dehydration may include:

• Thirst
• Headache
• Fatigue
• Dry mouth or lips
• Muscle cramps
• Infrequent urination and/or dark urine
• Increased heart rate
• Decreased appetite
• Lightheadedness, dizziness 

If dehydration remains unaddressed, it can progress to a more severe state, potentially resulting in symptoms such as:

• Rapid pulse
• Rapid breathing rate
• Feeling faint or fainting
• Confusion and/or delirium
• Lack of urination
• Lack of sweating
• Seizures

Acute and Chronic Conditions That May Contribute to Dehydration

• Heat exhaustion and heat stroke: These conditions can occur during prolonged exposure to extremely high temperatures. They may lead to a breakdown in the body's thermoregulation ability and severe dehydration. 
• Cognitive impairment and immobility: A common cause in elderly patients, reduced mobility and cognition can lead to inadequate dietary intake of fluids. 
• Severe burns: Burns can affect the skin's ability to protect against water loss, leading to greater dehydration risk. 
• Food-borne illnesses: Increased vomiting and diarrhea may lead to excess water loss and electrolyte imbalance. 
• Diabetes insipidus: In this condition, the kidneys may be unable to concentrate urine, leading to significantly increased urine output that can quickly lead to dehydration.
• Diabetes mellitus: High blood glucose levels can lead to increased urination as the body tries to eliminate the excess sugar through the kidneys.
• Inflammatory Bowel Disease (IBD): Ulcerative colitis and Crohn's disease can cause chronic diarrhea and increased vomiting, which may increase the risk of dehydration. Inflammation in the bowel can also lead to impaired water absorption from the colon. 
• Cystic fibrosis: In cystic fibrosis, chloride reabsorption may be impaired, leading to increased sweating, which can cause dehydration if fluids are not replenished. 

Lab Tests for Detecting Dehydration

Several lab tests can help detect dehydration:

Comprehensive Metabolic Panel 

A comprehensive metabolic panel (CMP) tests basic metabolic functioning and includes markers for electrolyte balance, kidney health, and liver health. Imbalances in some of these markers may indicate potential dehydration. 

Electrolytes are minerals that carry an electric charge when dissolved in water and blood. They are affected by hydration status as they help to balance the body's water levels. 

When water levels are too high, electrolyte concentrations can drop if not replenished. Conversely, if water levels are too low, then electrolyte concentrations rise. Therefore, increased levels of serum electrolytes on a CMP can signal possible dehydration. These include:

• Hypernatremia (elevated sodium levels)
• Hyperchloremia (elevated chloride levels)
• Hypercalcemia (elevated calcium levels)
• Hyperkalemia (elevated potassium levels)

As the kidneys are responsible for concentrating urine and filtering out and excreting waste products, decreased fluid intake can affect kidney function, leading to changes in kidney markers on a CMP:

• Elevated blood urea nitrogen (BUN): Urea is a metabolic waste product that is filtered and excreted by the kidneys. BUN may increase during dehydration due to a decrease in blood flow to the kidneys, which reduces overall filtration and excretion. 
• Elevated creatinine levels: Similar to BUN, creatinine is a waste product typically excreted by the kidneys, and its levels may increase with reduced water intake.
• Low estimated glomerular filtration rate (eGFR): eGFR is the kidney filtration rate. Reduced fluid intake may lead to less kidney filtration, lowering eGFR levels. 
• High BUN to creatinine ratio: When dehydrated, BUN may increase more than creatinine, creating a higher BUN level than creatinine. 

Albumin is a protein made by the liver and is present in the blood. It helps maintain proper oncotic pressure in blood plasma, balancing fluid levels between the vessels and tissues of the body. High serum albumin levels most often result from concentrated protein levels due to a decreased proportion of water in the blood from dehydration. 

Complete Blood Count

A complete blood count (CBC) is a routine blood test that evaluates the quantity and quality of red and white blood cells and their components. It involves analyzing a sample of whole blood, including red blood cells and plasma (the liquid component of blood). 

Dehydration may reduce the plasma volume, thereby increasing the concentration of red blood cells proportionally, resulting in:

• Elevated hemoglobin: The protein in red blood cells that binds and transports oxygen.
• Elevated hematocrit: The measurable proportion of red blood cells compared to total blood volume. 

Urinalysis

A urinalysis is a simple, non-invasive diagnostic tool that uses physical and chemical markers in the urine to detect conditions such as urinary tract infections and diabetes and to evaluate hydration status. 

The primary marker for dehydration is specific gravity, which evaluates urine concentration levels. As dehydration may lower the kidneys' filtering capacity, it can lead to a higher concentration of dissolved substances in the urine and increases specific gravity. 

Using Lab Values to Help Manage Dehydration

Diagnosis of dehydration requires a comprehensive approach, as no single lab test can definitively diagnose this condition. Many of the markers within the CBC, CMP, and urinalysis can be affected by other diseases and are not reliable on their own. 

Therefore, lab values are interpreted alongside presenting physical signs, symptoms, and relevant history to help determine hydration status. Physical signs of dehydration may include:

• Skin tenting
• Dry skin
• Dry mucous membranes, including the mouth and nasal passages
• Reduced sweating 
• Rapid heart rate
• Low blood pressure

Rehydration Strategies

Management depends on the severity level and cause of dehydration. For mild cases of dehydration, rehydration can be achieved at home by increasing oral intake of water and electrolytes to help replenish lost fluids. Signs and symptoms can be monitored for improvement to ensure hydration achievement.

More severe states may require medical attention and intravenous saline fluids to restore healthy hydration levels. These cases will often be monitored through lab work to confirm rehydration and may require routine follow-up when chronic conditions such as diabetes insipidus are present. 

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Key Takeaways

• Crucial labs for assessing dehydration include:
  • Comprehensive Metabolic Panel (CMP): Elevated sodium, chloride, calcium, and potassium may indicate dehydration. Raised BUN, creatinine, and high BUN to creatinine ratio may reflect kidney stress due to low fluid intake. High albumin may indicate water loss. 
  • Complete Blood Count (CBC): Elevated hemoglobin and hematocrit may suggest reduced plasma volume from dehydration.
  • Urinalysis: High specific gravity may indicate concentrated urine, a key marker of dehydration.
• Recognizing dehydration early allows for prompt management, potentially preventing severe health issues such as confusion, rapid pulse, and seizures.
• Accurate diagnosis using lab tests and symptom evaluation can help ensure effective rehydration strategies.
• If experiencing symptoms like intense thirst, dizziness, or rapid heartbeat, consider seeking medical help immediately.
• Early intervention can help avert serious complications, particularly in vulnerable populations like the elderly.