Postural Orthostatic Tachycardia Syndrome (POTS) is a disorder of the autonomic nervous system, which controls automatic functions like heart rate, blood pressure, and digestion. The defining feature of POTS is an abnormal increase in heart rate that occurs immediately upon standing upright. Individuals frequently experience lightheadedness, fatigue, and palpitations when transitioning from lying to standing. While blood work is routinely ordered during the diagnostic process, it serves a supportive role rather than acting as a direct test for the syndrome itself.
Establishing a POTS Diagnosis
POTS cannot be confirmed solely by analyzing blood samples. The definitive diagnosis relies on specific clinical criteria and objective physiological testing. Diagnosis requires the patient to have experienced symptoms of orthostatic intolerance for at least three to six months, which must significantly improve when the patient reclines.
The objective requirement is a sustained increase in heart rate when moving to an upright posture. For adults, this increase must be 30 beats per minute or more within 10 minutes of standing. For adolescents (12 to 19 years old), the threshold is 40 beats per minute or more.
The primary method for documenting this response is the Tilt Table Test or the Active Stand Test. These tests monitor heart rate and blood pressure while the patient moves from lying to upright. Crucially, the heart rate increase must occur without a significant drop in blood pressure (a systolic drop of 20 mmHg or less), which distinguishes POTS from orthostatic hypotension.
Using Blood Work to Rule Out Other Conditions
Routine blood work serves the primary function of differential diagnosis in the POTS workup. This testing is necessary because many conditions share symptoms like fatigue, dizziness, and rapid heart rate, which can be mistaken for POTS. By ruling out these other possible causes, the medical team confirms the patient’s symptoms are not better explained by a treatable, non-autonomic disorder.
Standard Screening Tests
Standard screening tests are performed to identify common conditions that mimic or exacerbate orthostatic symptoms:
- A Complete Blood Count (CBC) screens for anemia, a common cause of fatigue and lightheadedness, which can mimic orthostatic symptoms.
- Thyroid function tests, measuring hormones like Thyroid-Stimulating Hormone (TSH), exclude hyperthyroidism or hypothyroidism, both of which affect heart rate and energy levels.
- The Comprehensive Metabolic Panel (CMP) assesses kidney function and electrolyte balance. Abnormalities in electrolytes like sodium and potassium impact cardiovascular regulation and can cause orthostatic intolerance.
- Blood tests for vitamin deficiencies, particularly Iron/Ferritin and Vitamin B12, are standard practice because low levels of these nutrients are frequently associated with debilitating fatigue.
- Screening for blood sugar regulation through a Glucose or Hemoglobin A1C (HbA1C) test rules out diabetes or hypoglycemia, conditions that can cause autonomic neuropathy.
Specialized Blood Tests for Underlying Causes
Beyond routine screening, specialized blood tests are often utilized to investigate potential underlying causes or to classify the specific subtype of POTS. These tests are typically ordered after all common alternative diagnoses have been eliminated.
Norepinephrine and Volume Status
One specialized test measures plasma norepinephrine levels, taken while the patient is lying down and again while standing upright. Norepinephrine is a neurotransmitter that helps regulate the body’s “fight or flight” response. Its levels are often significantly elevated in patients with hyperadrenergic POTS. If the standing plasma norepinephrine level is found to be above approximately 600 picograms per milliliter, it provides evidence for this specific subtype.
Other specialized hormone panels, such as those measuring Renin and Aldosterone, assess the patient’s plasma volume status. Low plasma volume, or hypovolemia, is a common feature in many POTS patients, and the levels of these hormones provide insight into the body’s ability to regulate fluid and salt balance.
Autoimmune and Inflammatory Markers
In cases where an autoimmune etiology is suspected, specific autoantibodies can be tested, though this remains highly specialized. Antibodies targeting adrenergic receptors are being studied for their potential role in causing autonomic dysfunction in some individuals.
Additionally, blood tests for markers of Mast Cell Activation Syndrome (MCAS) may be conducted for patients who experience symptoms suggestive of allergic or inflammatory reactions. These tests often measure serum tryptase, a chemical released by mast cells, to determine if this co-occurring condition is contributing to the patient’s overall symptom presentation.