Dehydration is defined as a deficit of total body water, occurring when fluid intake is less than fluid loss. This common condition significantly impacts the results of diagnostic tests performed on blood samples. Since blood work is an important tool for monitoring health, proper preparation is necessary to ensure accurate readings. Ignoring hydration status before a blood draw can lead to false interpretations, potentially resulting in unnecessary follow-up tests or an incorrect diagnosis.
How Dehydration Alters Blood Composition
The fundamental mechanism by which a lack of water affects blood test results is known as hemoconcentration. Blood is composed of plasma (the fluid portion) and a cellular portion, which includes red and white blood cells and platelets. When the body becomes dehydrated, the volume of water in the plasma decreases considerably.
This reduction in plasma volume effectively concentrates the remaining components within the blood sample. Although the absolute number of cellular elements and solutes remains unchanged, their concentration relative to the reduced fluid volume increases.
The resulting hemoconcentration causes the measurement of various substances to appear falsely elevated on a lab report. This artificial increase can lead a healthcare provider to believe a component is higher than it is in reality, suggesting an illness that is not present. Markers of health become unreliable indicators until proper hydration is restored.
Common Blood Tests Skewed by Dehydration
Hematology Markers
Dehydration directly impacts the measurements of red blood cell components, such as Hematocrit (Hct) and Hemoglobin (Hgb). Hct measures the percentage of red blood cells in the total blood volume, while Hgb measures the oxygen-carrying protein within those cells. Since the fluid component of the blood shrinks, the fixed amount of red blood cells becomes concentrated, artificially raising both Hct and Hgb values.
A falsely elevated hematocrit, for instance, might be interpreted as polycythemia, a condition characterized by an abnormal increase in red blood cells. Conversely, in a patient with anemia, dehydration could temporarily mask the severity of their low red blood cell count by making the values appear closer to the normal range.
Kidney Function Markers
Key indicators of kidney health, Blood Urea Nitrogen (BUN) and Creatinine, are highly sensitive to hydration status. Both are waste products filtered by the kidneys, and elevated levels can signal a problem with kidney function.
Dehydration reduces blood flow to the kidneys, impairing their ability to filter waste products efficiently, causing them to build up in the bloodstream. BUN is particularly susceptible and often rises disproportionately to creatinine. Healthcare providers recognize this pattern as a strong indicator of dehydration rather than primary kidney disease.
Electrolytes
Electrolytes, such as Sodium and Potassium, are minerals that carry an electrical charge necessary for nerve and muscle function. Dehydration can cause complex shifts in their balance, often leading to a perceived increase in sodium concentration (hypernatremia).
The concentration of these markers can create an inaccurate picture of the body’s true electrolyte balance. An elevated sodium level may be a direct result of hemoconcentration, rather than a severe imbalance requiring medical intervention.
Glucose and Protein
Blood glucose levels can be affected by a lack of proper hydration. Dehydration can lead to a false increase in blood glucose, which might complicate the interpretation of tests for diabetes or pre-diabetes.
Similarly, total protein and albumin levels, which reflect nutritional and liver status, can appear artificially high. This occurs because the proteins are contained within the reduced plasma volume, yielding a concentrated measurement.
Guidelines for Hydration Before Blood Work
Patients can take simple steps to ensure their hydration status does not compromise the accuracy of their blood test results. The most straightforward recommendation is to increase water intake in the 24 hours leading up to the scheduled appointment. This helps maintain normal blood volume, preventing hemoconcentration.
For tests requiring fasting (such as glucose or cholesterol), it is generally permissible and encouraged to drink plain water, even during the fasting period. Drinking water keeps the blood less viscous, which also benefits the technician by making veins more visible and easier to access for a smoother blood draw.
It is prudent to avoid substances known to promote fluid loss, such as excessive caffeine and alcohol, for at least 24 hours before the test. These substances have diuretic properties that can lead to dehydration.
If a patient believes they were dehydrated at the time of the blood draw (due to illness or insufficient fluid intake), they should notify their healthcare provider. The provider can then take the patient’s hydration status into account when interpreting the results or recommend a retest.