Red blood cells (RBCs) are the most abundant cells in the bloodstream, transporting oxygen from the lungs to the body’s tissues and carrying carbon dioxide away. A standard blood test, the Complete Blood Count, measures the red blood cell count, indicating the number of these cells in a specific volume of blood. An elevated RBC count on a lab report can be concerning, but it does not always signal a serious underlying disorder. One common and temporary explanation for this elevated reading is the body’s fluid balance, specifically dehydration.
Confirming the Link: Dehydration and Elevated RBC Readings
Dehydration can cause a temporary increase in the measured concentration of red blood cells in the bloodstream. This effect is often reflected in an elevated hematocrit (Hct) value, which is the percentage of blood volume made up by red cells. When less fluid is available, the components of the blood become more densely packed together. This phenomenon is a common, non-disease-related reason for a high reading following a routine blood draw. Recognizing this immediate link is important before investigating more complex medical explanations for an altered blood cell count.
The Mechanism: How Hemoconcentration Distorts Blood Counts
The process that leads to this false high reading is known as hemoconcentration. Hemoconcentration occurs because dehydration reduces the volume of plasma, the liquid component of blood primarily composed of water. When dehydrated, fluid shifts out of the bloodstream to maintain balance in other tissues. The total number of red blood cells circulating remains unchanged during this process.
Since the liquid volume they are suspended in has decreased, the cells are concentrated into a smaller space. This situation is similar to concentrated orange juice: the amount of pulp has not changed, but the ratio of pulp to liquid is much higher. A blood test measures this ratio. The reduced plasma volume makes it appear as though there is an excess of red blood cells, showing a higher concentration even though the body has not produced any extra cells.
Relative Versus Absolute Polycythemia
Understanding the distinction between two types of high red blood cell counts is important for interpreting blood test results. The temporary increase caused by dehydration is classified as relative polycythemia, sometimes called spurious polycythemia. This condition reflects a concentrated state of the blood due to plasma loss, rather than an actual increase in the cell population. Relative polycythemia is a transient finding that resolves once normal fluid balance is restored through rehydration.
In contrast, absolute polycythemia involves an actual, sustained increase in the total mass of red blood cells. This condition results from the bone marrow producing too many red blood cells, often in response to chronic low oxygen levels or an underlying blood disorder. Absolute polycythemia represents a true biological excess of cells, while the relative form is merely a measurement artifact caused by low fluid volume.
When to Seek Medical Follow-Up
If a blood test reveals an elevated red blood cell count and dehydration is suspected, the initial step is often to rehydrate properly and then repeat the blood work. Restoring fluid status should normalize the plasma volume, causing the measured concentration to return to a typical range. If the elevated count persists even after rehydration, further investigation is necessary to rule out absolute polycythemia or other medical conditions.
You should seek medical attention if a high count is accompanied by symptoms such as persistent headaches, dizziness, blurred vision, or unexplained fatigue. These symptoms, especially when combined with a consistently high reading, may indicate a more serious issue requiring a physician’s evaluation. Determining whether the cause is a simple fluid imbalance or increased cell production requires a detailed medical history and often additional testing.