CPR First Aid

Effects of Dehydration on Blood Test Result

Effects of Dehydration on Blood Test Result

Dehydration can have a substantial impact on the findings of blood tests. A falsely high reading on some common blood tests may result from the body’s decreased blood volume when it is dehydrated. As a result, if you are dehydrated, your doctor may believe you have a more dangerous condition than you do. This blog post will go through the effects of dehydration on blood test result, and ways to avoid dehydration’s negative impact on blood test results.

What is Dehydration?

The excessive loss of bodily fluids is dehydration. Dehydration occurs when we lose more fluid than we can replenish. Thirst typically serves as a warning indicator of fluid depletion or water loss, and drinking can restore it. 

"Bodily fluid loss leads to dehydration. Thirst signals fluid depletion; drinking restores."

Dehydration Prevalence in Australia

According to Couriermail, 80% of Australians suffer from symptoms typical of dehydration and the majority don’t recognise key symptoms. The most common symptoms affecting Australians are lethargy (62%) and loss of energy (51%). The research also showed that less than one in 10 Australians (6%) associate problems with concentration and mental impairment with dehydration, despite it being a common symptom for most (56%).

Causes of Dehydration

Possible causes of dehydration include:

  • Excessive perspiration after engaging in strenuous exercise (especially in hot weather).
  • Not drinking enough fluids, especially in hot weather.
  • Diarrhoea or vomiting
  • Fever
  • Drinking too much alcohol
  • Diabetic complications
  • Taking of diuretics
  • Fluid loss due to health conditions such as diabetes insipidus, glucosuria, and others

Symptoms of Dehydration

Learn more about how to see if your dehydrated condition is serious by checking these symptoms:

  • Extreme thirst
  • Urinating less than usual
  • Headache
  • Dark-coloured urine
  • Sluggishness and fatigue
  • Dry mouth
  • Dizziness, light-headedness
  • Confusion
  • Faster than normal heart rate
  • Low blood pressure
  • Orthostatic hypotension

Complications of Dehydration

Complications of severe dehydration can include:

  • Kidney damage
  • Seizures
  • Shock due to low blood volume
  • Sudden and unexplained weight loss

Diagnosis of Dehydration

Correct diagnosis of dehydration is done through the following:

  • Checking of blood samples to see if there is electrolyte imbalance (through chloride potassium and sodium levels)
    -Remember to avoid smoking or vaping before a blood draw as nicotine may constrict the blood vessels, making it difficult to insert a needle into a vein
  • Urinalysis which also checks for signs of a bladder infection

How Can You Avoid Dehydration?

Healthcare providers and gov agencies state that rehydration is important, especially if you’re in a hot environment, exercising hard, or have diarrhoea.

According to healthdirect, the amount of water that each individual should consume varies substantially. It depends on your particular metabolism, the weather, what you consume, your age, and any medical conditions you may have. Kids, older adults, and seniors must get enough water to stay healthy.

About one-fifth of the water comes from meals, with the remainder coming from liquids. Throughout the day, the body eliminates water through breathing, sweating, and restroom use.

Men typically need approximately 10 cups of water each day, while women typically need about 8 cups (add another cup a day if you are pregnant or breastfeeding). Babies need 0.7 to 0.8 litres of fluid a day from breast milk or formula, while children need between 4 cups (for 1-year-olds) and 6 to 8 cups a day (for teenagers). In Australia, 1 cup is equivalent to 250ml.

Any liquid, including tea, coffee, fruit juice, and soft drinks, contains water. But watch how much of them you consume because they can cause weight gain, harm your teeth, and have an unwelcome stimulating effect. The best option is always Australian tap water.

Stay hydrated: crucial in heat, exercise, or diarrhoea.

Effects of Dehydration on Blood Test Result

According to Sprint Medical, the following blood tests are affected when you’re dehydrated.

Hematocrit is the volume of Red Blood Cells (RBCs) in the blood. Dehydration is the most common cause of a high hematocrit value because the volume of the fluid in the blood drops and the red blood cells per volume of blood rises, thus causing inaccurate results of the test. However, with adequate intake of fluid, the hematocrit returns to normal.

  1. Effect of Dehydration on Hemoglobin
    If a person is severely dehydrated, the haemoglobin will appear higher than the patient with normal blood volume. Also, if the patient’s fluid is overloaded, the haemoglobin range will be lower than their actual level. This is because haemoglobin and hematocrit are based on whole blood volume and therefore are dependent on plasma volume.
  2. Effect of dehydration on CBG measurement
    Dehydration causes hypertension and increases glucose utilization in the local tissues. Thus this leads to the false low result of the capillary blood glucose (CBG) test.
  3. Effect of Dehydration on Blood Glucose
    The rise in glucose was found to be out of proportion to changes in metabolite concentrations that could be due to the passive concentration of the plasma (haemoconcentration) as a result of dehydration. Also, an increase in hepatic glucose production, with increased plasma glucose levels during hyperosmolality can be caused by dehydration.
  4. Effect of Dehydration on Renal Function Test
    Dehydration has multiple effects on the kidneys. The loss of body water leads to an increase in serum osmolality and activation of vasopressin which results in urinary concentration.
  5. Effect of Dehydration on Liver Function Test
    There is a significant difference in serum total albumin and protein levels in a dehydrated patient before and after hydration with intravenous fluid. Still, the liver enzymes and bilirubin levels remain unchanged, indicating the change in protein level is due to dehydration status rather than a liver abnormality.
  6. Effect of Dehydration on Lipid Profile
    There has been an investigation carried out on fasting subjects to see dehydration on lipid profiles. The subjects were allowed to fast firstly with no fluid replacement and then with salt and water supplements where the fasting subjects with no fluid had higher total serum cholesterol, LDL cholesterol, HDL cholesterol, apolipoprotein A1, and apolipoprotein B compared to the subjects who had fasted with salt supplements and prior fluids.
  7. Effect of Dehydration in Cholesterol Test
    Dehydration causes blood volume to decrease, leading to a drop in blood pressure and blood flow, resulting in cholesterol accumulation in the arteries. To protect cell membranes, the body may increase the production of cholesterol in response to severe dehydration.
  8. Effect of Dehydration in a Lab Blood test and Blood pressure
    Due to dehydration, there can be inaccurate blood test results as suspected, as water accounts for approximately 60% of the adult human body weight. So, lack of water may lead to dehydration and thus affect blood pressure and blood test results.
  9. Effect of Dehydration in Kidney Value Test
    Two of the most common tests for diagnosing kidney diseases and evaluating kidney functions are the creatinine test and the blood urea nitrogen (BUN) test. Higher volumes of BUN and creatinine indicate that the kidney is not working properly as the test measures the amount of waste product in the blood. Dehydration can reduce the blood flow to the kidney and can elevate the blood urea nitrogen level and creatinine level in the blood.

Why Do You Need to Fast Before Blood Tests?

Fasting before certain blood tests is often necessary to ensure accurate results. The main reason for fasting is to obtain baseline measurements of various substances in the blood without the influence of recent food intake. 

Fasting is typically required before tests such as fasting blood sugar (FBS) or fasting plasma glucose (FPG) tests. This is because eating or drinking anything can raise blood sugar levels, potentially leading to a misdiagnosis of diabetes or other glucose-related disorders.

Fasting before blood tests helps ensure that the results accurately reflect baseline levels of various substances in the blood. With the fasting period being long, the question many have is, can you drink water with a fasting blood test? Let’s find out.

How Do You Fast Before Blood Tests?

Fasting before blood tests involves abstaining from all food and beverages, except water, for a specified period leading up to the test. 

Typically, fasting for 8 to 12 hours is recommended before blood tests that require fasting. Your healthcare provider will specify the duration of fasting required based on the specific tests being performed.

Finally, while fasting, try to stay relaxed and avoid strenuous physical activity, as this can affect blood sugar and other metabolic parameters.

How Soon Should You Drink Water Before a Blood Test?

Drinking water helps maintain hydration levels, which is important for a successful blood draw and accurate test results. Dehydration can lead to difficulties in locating veins and obtaining an adequate blood sample.

You can drink water freely during the fasting period before the blood test, unless otherwise instructed by your healthcare provider. Water consumption helps prevent dehydration and ensures that you remain hydrated throughout the fasting period.

While it’s important to stay hydrated, avoid excessive water intake immediately before the blood test, as this may lead to overhydration and dilution of blood samples, potentially affecting test accuracy.

First Aid Course

If you need a general First Aid qualification, we recommend the HLTAID011 Provide first aid. This also includes CPR qualification. The Australian Resuscitation Council (ARC) recommends that this qualification be updated every 3 years.

If you are looking to update your CPR qualification then we recommend HLTAID009 Provide cardiopulmonary resuscitation.  The Australian Resuscitation Council (ARC) recommends that this qualification be updated annually.

If you are looking for a first aid qualification to work in the childcare industry, and also require Anaphylaxis and Asthma training ONLY, then you should consider HLTAID012 Provide an emergency first aid response in an education and care setting.

If you are working with children, you may also be required to obtain first aid training qualifications in Asthma and Anaphylaxis.  You need to check with the employer before enrolling so that the client gets the correct training for his or her circumstance.

Please note that you still need to confirm with your employer first what qualification you need to take before you book.

In Australia, workplaces must follow occupational health and safety regulations – amongst others, these specify that businesses and care organisations must have personnel on staff with current, recognised workplace-approved CPR and first aid training.

Click here to view a full list of CPR First Aid courses currently available on RTO’s scope of registration.

Click here to find CPR and first aid courses near you.


Dehydration can therefore be risky and life-threatening if it is not treated quickly, and it can also negatively impact the patient’s condition and laboratory findings. Therefore, a patient should avoid unneeded hot, dry environments, physical activities, and diuretic medications like caffeine before any routine blood test and make sure to drink enough water



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