Blood tests provide a quantitative snapshot of a person’s internal biochemistry and overall physiological state. When fighting an acute illness, such as a common cold or a mild viral infection, the body initiates a complex response that directly alters the composition of the blood. Therefore, being sick definitively affects blood work results. Understanding why these temporary changes occur is necessary to interpret lab results accurately, especially when tests are performed for routine health monitoring rather than urgent diagnosis.
How Acute Illness Alters Your Body’s Internal Environment
The body’s defense against an invading pathogen involves a physiological stress response. This reaction is orchestrated by the immune and endocrine systems, causing temporary shifts in blood composition. The release of chemical messengers known as cytokines is a fundamental component of this process.
Cytokines are signaling proteins released by immune cells that coordinate the inflammatory response, which then triggers the liver to produce acute-phase reactants. This systemic inflammation is designed to fight the infection but creates an environment far from the body’s normal baseline.
Illness often disrupts fluid balance, leading to changes in hydration status. Symptoms like fever, vomiting, or diarrhea cause fluid loss, decreasing the overall plasma volume in the blood. When the fluid portion decreases, the remaining components become more concentrated, potentially leading to falsely elevated readings for various markers.
The fight-or-flight response is activated during sickness, leading to a surge in stress hormones like cortisol. Cortisol regulates metabolism, and its increased presence can cause a temporary rise in blood glucose levels. This hormonal fluctuation provides the body with immediate energy to combat the stressor.
Key Blood Tests Affected by Temporary Sickness
Acute illness causes predictable changes in the Complete Blood Count (CBC) and inflammatory markers. The total White Blood Cell (WBC) count almost always elevates as the immune system mobilizes defenses. The differential count, which measures the percentages of different WBC types, offers a clue about the kind of infection present.
Bacterial infections typically cause a marked increase in neutrophils, often accompanied by a “left shift,” indicating the rapid release of immature neutrophils from the bone marrow. Conversely, acute viral infections, like the common cold, often cause a higher proportion of lymphocytes to fight the virus. These shifts are a normal, temporary response, but they can easily lead to misinterpretation if the patient’s sick status is unknown.
Inflammatory markers show the most dramatic and rapid changes during an acute illness. C-reactive protein (CRP) is a protein produced by the liver that can increase dramatically within hours of an infection starting. The Erythrocyte Sedimentation Rate (ESR) also rises, though more slowly, and can take weeks to normalize after the illness resolves because it is affected by persistent changes in plasma proteins.
The metabolic panel, which includes tests for liver and kidney function, can be temporarily skewed. Dehydration, common with fever or vomiting, can lead to transient increases in kidney function markers such as creatinine and blood urea nitrogen (BUN). The stress-induced increase in cortisol can also elevate blood glucose levels, potentially suggesting pre-diabetes or diabetes, even if the person is metabolically healthy at baseline.
Timing and Preparation for Accurate Blood Work
The decision to proceed with scheduled blood work when feeling unwell depends on the purpose of the test. For routine annual checkups or non-urgent monitoring, rescheduling is generally the best approach to ensure accurate baseline results. Waiting approximately one to two weeks after all symptoms have resolved allows inflammatory markers and cell counts to return to their normal physiological levels.
If the blood test is ordered specifically to diagnose or manage the current acute illness, the test should be done immediately. In this diagnostic scenario, the elevated markers are the very information the healthcare provider needs to determine the type and severity of the infection. Clearly communicate the illness, including symptoms like fever or vomiting, to both the ordering physician and the phlebotomist drawing the blood.
For all testing, preparation should focus on optimizing hydration, as dehydration can concentrate the blood and falsely elevate several markers. Even if fasting is required for a test, drinking plain water is permissible and encouraged. Transparent communication about any medications, including over-the-counter fever reducers or decongestants, is necessary, as these can sometimes interfere with liver or kidney enzyme results.