A nitric oxide blood test measures the breakdown products of nitric oxide (NO) in your blood, not the molecule itself. Because NO disappears within seconds of being produced, labs instead quantify what it leaves behind: nitrite and nitrate, collectively called NOx. These metabolite levels serve as a proxy for how much nitric oxide your body is making and how well your blood vessels are functioning. The test is most often ordered to evaluate endothelial function, assess cardiovascular risk, or investigate conditions where NO production has gone haywire, but interpreting the results demands more care than a standard blood panel because everything from last night’s salad to your morning mouthwash can throw off the numbers.
Why Nitric Oxide Matters to Your Cardiovascular System
Nitric oxide is one of the most powerful signaling molecules in your body. Produced primarily by the enzyme endothelial nitric oxide synthase (eNOS) in the cells lining your blood vessels, NO relaxes smooth muscle, widens arteries, and keeps blood pressure in check. Research in mice engineered to lack the eNOS gene shows elevated blood pressures and heart rates, confirming that eNOS is essential for normal cardiovascular control.1PubMed. Elevated blood pressures in mice lacking endothelial nitric oxide synthase In humans, studies using a drug that blocks nitric oxide production found that NO tonically restrains blood pressure by roughly 30 mm Hg, making it one of the most potent metabolic regulators of blood pressure we know of.2PubMed. Contribution of endothelial nitric oxide to blood pressure in humans
When your body loses the ability to produce adequate NO, the earliest stage of cardiovascular trouble has already begun. This loss, often called endothelial dysfunction, is considered the opening event in the development of hypertension and atherosclerosis.3PubMed. Nitric oxide deficiency is a primary driver of hypertension That is precisely why a test that gauges NO production can be clinically useful: it offers a window into vascular health before overt disease sets in.
What the Test Actually Measures
NO has an extremely short half-life in blood, often vanishing in under a second. You cannot simply draw a vial of blood and measure nitric oxide directly. Instead, the lab measures what NO turns into once it reacts with oxygen, hemoglobin, and surrounding tissue. The two primary metabolites are nitrite (NO₂⁻) and nitrate (NO₃⁻). Quantifying these metabolites in blood samples provides valuable information about in vivo NO production and bioavailability.4PubMed Central. Methods to detect nitric oxide and its metabolites in biological samples
Of the two metabolites, nitrite is more closely tied to recent, local NO activity, while nitrate reflects longer-term, systemic production. Most clinical labs report a combined NOx value (nitrite plus nitrate together), though some specialized research labs report them separately. One widely used method relies on the Griess reaction, which converts nitrite into a colored compound that can be measured with a standard light-based instrument. Because most of the NO in blood ultimately becomes nitrate, labs first use an enzyme to convert nitrate back to nitrite before running the color reaction. This approach has a reported recovery rate of about 88%.5PubMed. Determination of nitrite/nitrate in human biological material by the simple Griess reaction
Another important technique is chemiluminescence, which detects the light emitted when NO reacts with ozone. Chemiluminescence is more sensitive than the Griess method and can pick up smaller changes, which is why many research labs prefer it. For routine clinical work, though, Griess-based assays remain common because they are cheaper and easier to run on standard equipment.
Preparing for the Test
Here is where nitric oxide testing diverges sharply from most blood work. A standard cholesterol or glucose test asks you to fast overnight. A nitric oxide test asks you to control your entire diet for days beforehand, because everyday foods contain far more nitrate than your body produces on its own.
Normal food contains more nitrate than what the body forms from NO, meaning diet-derived nitrate can dominate the blood concentration and mask the signal from your own endothelial production.6PubMed. Nitric oxide as a challenge for the clinical chemistry laboratory A single serving of nitrate-rich food (think beets, leafy greens, cured meats) can spike plasma nitrate from around 30 micromoles per liter to over 200, a roughly sevenfold jump, within an hour.7PubMed. Plasma nitrate as an index of nitric oxide formation in man: analyses of kinetics and confounding factors That kind of dietary noise easily swamps whatever your blood vessels are contributing.
Research consistently shows that you need to restrict nitrate and nitrite intake for at least 48 hours before the blood draw for plasma nitrate to stabilize at a level that reflects endogenous production.7PubMed. Plasma nitrate as an index of nitric oxide formation in man: analyses of kinetics and confounding factors Some investigators have found that the variation between and within subjects does not settle fully until the fourth day of a low-nitrate diet, and have recommended at least four days of dietary restriction for the most reliable readings.8PubMed. Effects of diet on measurement of nitric oxide metabolites In practice, your doctor or the ordering lab should give you a list of high-nitrate foods to avoid. Common culprits include beets, spinach, arugula, celery, radishes, and processed meats containing sodium nitrate as a preservative.
The Mouthwash Problem
One of the stranger confounders in nitric oxide testing involves bacteria living on your tongue. Your salivary glands actively concentrate nitrate from the blood, creating saliva with nitrate levels many times higher than plasma. Bacteria on the back of the tongue then convert that salivary nitrate into nitrite, which gets swallowed and re-enters the bloodstream. This recycling loop is a meaningful contributor to your circulating nitrite levels.
When researchers had healthy volunteers use antiseptic mouthwash, oral nitrite production dropped by about 90%, and plasma nitrite fell by roughly 25%.9PubMed Central. Physiological role for nitrate-reducing oral bacteria in blood pressure control That means a quarter of the nitrite in your blood may depend on whether your mouth bacteria are intact. If you have been using a strong antibacterial mouthwash in the days before a nitric oxide test, your results could read falsely low. Some labs now advise patients to avoid antibacterial mouthwash for a defined period before testing, though this recommendation is not yet universal.
What Low NOx Levels Suggest
The most common clinical reason for ordering a nitric oxide blood test is suspicion of endothelial dysfunction, and low NOx values tend to confirm it. People with essential hypertension have significantly lower combined plasma nitrate-plus-nitrite concentrations compared to people with normal blood pressure. In one study, hypertensive individuals averaged about 15.7 micromoles per liter compared with 22.8 in normotensive controls, and the plasma NOx concentration showed an inverse relationship with both systolic and diastolic blood pressure.10PubMed. Reduced plasma concentrations of nitrogen oxide in individuals with essential hypertension People with borderline hypertension fell between the two groups, suggesting a gradient rather than a clean cutoff.
Patients with cardiovascular risk factors and confirmed endothelial dysfunction also show decreased levels of circulating nitroso compounds, another family of NO-related molecules. These may eventually serve as diagnostically useful markers of NO bioavailability and a surrogate measure of endothelial function.11PubMed. Plasma nitroso compounds are decreased in patients with endothelial dysfunction
Diabetes is another context where low NOx values carry meaning. Diabetes impairs eNOS activity while also boosting production of reactive oxygen species that scavenge NO before it can act, resulting in diminished NO bioavailability and the vascular damage that follows.12Journal of the American College of Cardiology. Diabetes Mellitus-Associated Vascular Impairment: Novel Circulating Biomarkers and Therapeutic Approaches In type 2 diabetic patients with kidney complications, the actual synthesis of NOx from arginine is reduced under both fasting and insulin-stimulated conditions, extending the concept of insulin resistance to NO metabolism itself.13PubMed Central. Nitric oxide synthesis is reduced in subjects with type 2 diabetes and nephropathy
What Elevated NOx Levels Suggest
While low NO levels point toward chronic vascular problems, high NOx values can signal acute, dangerous inflammation. Sepsis is the classic example. When the immune system goes into overdrive during a severe infection, a different form of nitric oxide synthase (the inducible form, iNOS) ramps up in immune cells and produces massive amounts of NO. This flood of NO dilates blood vessels so aggressively that blood pressure can plummet. Septic patients show significantly elevated plasma nitrite and nitrate levels, and these high NOx values track with low vascular resistance and elevated endotoxin.14PubMed Central. Nitrogen oxide levels in patients after trauma and during sepsis
Pregnancy produces its own distinctive pattern. Normal pregnancy raises plasma NOx concentrations compared to the non-pregnant state, reflecting the vasodilation that supports the placenta. In one study, normotensive pregnant women averaged about 30 micromoles per liter, up from 18 in non-pregnant controls. Women with preeclampsia had even higher levels, around 45.6 micromoles per liter, with a positive correlation to systolic blood pressure. But pregnant women with underlying essential hypertension showed markedly lower levels, around 19 micromoles per liter, similar to non-pregnant values.15Gynecologic and Obstetric Investigation. Plasma Nitric Oxide Levels in Pregnant Patients with Preeclampsia and Essential Hypertension That split between preeclampsia and chronic hypertension during pregnancy is a genuinely useful diagnostic distinction.
Other Factors That Shift Your Results
Beyond diet and mouthwash, several medications influence NOx levels. Common blood pressure drugs including ACE inhibitors, calcium channel blockers, and newer vasodilating beta-blockers all enhance NO bioactivity. Statins do the same, both through their cholesterol-lowering effect and through independent mechanisms.16PubMed Central. Targeting nitric oxide with drug therapy If you are taking any of these, your NOx levels could look better than your underlying endothelial function would warrant. Your doctor needs to know your full medication list when interpreting the results.
Exercise is another powerful influence. A meta-analysis of exercise training studies found that both moderate aerobic exercise and high-intensity interval training significantly increase NO and NOx levels, with the effect appearing within eight weeks and persisting with continued training.17Frontiers in Physiology. Effect of exercise training on nitric oxide and nitrate/nitrite (NOx) production: A systematic review and meta-analysis A vigorous workout before your blood draw could temporarily boost NOx levels.
Age and sex matter too. In a study of healthy non-smokers, men aged 20 to 29 had higher NOx levels than women of the same age. Both sexes saw NOx concentrations peak between the ages of 50 and 59.18PubMed. Serum nitric oxide metabolite levels in a general healthy population: relation to sex and age The sex gap in middle age appears to have less to do with biological sex per se and more to do with menopause: research suggests that the difference in serum NOx between middle-aged men and women is more closely related to menopausal status than to gender itself.19PubMed. Menopause status as the main factor explaining the gender differences of serum nitric oxide concentrations in middle-aged population Time of day also plays a role. Circadian studies in healthy adults have documented day-to-night variation in blood NO levels, meaning morning and evening draws could yield different numbers from the same person.20PubMed. Day-night variations in blood levels of nitric oxide, T-TFPI, and E-selectin
Blood Test Versus Exhaled Nitric Oxide
If you have asthma, you may have encountered a different kind of nitric oxide test entirely: fractional exhaled nitric oxide (FeNO). That test measures NO in your breath, not your blood, and it serves a completely different purpose. FeNO reflects inflammation in the airways, particularly eosinophilic inflammation, and is used to guide asthma therapy. It has a predictive relationship with sputum eosinophils but does not correlate well with blood eosinophil percentages.21PubMed Central. Association between fractional exhaled nitric oxide, sputum induction and peripheral blood eosinophil in uncontrolled asthma
The two tests are not interchangeable. A blood NOx test tells you about systemic vascular NO production. A FeNO test tells you about airway inflammation. Having abnormal results on one says nothing about the other. If your doctor ordered a FeNO test, the dietary restrictions and mouthwash precautions discussed in this article do not apply in the same way; FeNO testing has its own protocol. Confusing the two tests is one of the more common misunderstandings patients bring to their appointments.
Salivary Test Strips and At-Home Monitoring
You may have seen nitric oxide test strips sold online, marketed as a convenient way to check your NO status at home. These strips are placed on the tongue and measure salivary nitrite through a color change. They are inexpensive and provide an instant reading, but their accuracy has important limits.
Validation studies show that commercial salivary strips correlate reasonably well with lab-measured salivary nitrite concentrations, with correlation coefficients in the 0.59 to 0.83 range depending on the brand and the lab method used for comparison.22PubMed Central. Validation of two Point-of-care Tests against Standard Lab Measures of NO in Saliva and in Serum However, neither brand tested in one validation study showed any significant correlation with serum nitrite or nitrate levels.22PubMed Central. Validation of two Point-of-care Tests against Standard Lab Measures of NO in Saliva and in Serum That is the central limitation: salivary nitrite and plasma nitrite do not reliably track each other. Plasma nitrate does correlate with salivary nitrate and with salivary nitrite, but plasma nitrite itself does not correlate with salivary nitrite.23PubMed Central. The Relationship between Plasma and Salivary NOx
Further research confirms that while the strips are decent at tracking relative changes in salivary nitrite (the direction is usually right), they do not provide accurate absolute concentrations.24PubMed. Validity and reliability of test strips for the measurement of salivary nitrite concentration with and without the use of mouthwash in healthy adults So if you are using the strips to see whether a dietary change or exercise program is moving your NO levels in the right direction, they can offer a rough signal. If you are trying to diagnose endothelial dysfunction or get a number you can compare to clinical reference ranges, the strips are not the right tool.
Where the Technology Is Headed
The gap between what researchers need and what current tests can deliver has pushed development of new sensor technology. Microfluidic sensors that can analyze NO directly in small whole-blood samples (as little as 250 microliters, about five drops) have been demonstrated in laboratory settings.25PubMed Central. Microfluidic Amperometric Sensor for Analysis of Nitric Oxide in Whole Blood These bypass the need to measure metabolites at all by detecting NO itself as it is produced.
Even more futuristic designs include flexible, biodegradable wireless sensors intended for implantation. One prototype reported a detection limit of about 4 nanomoles and a sensing range spanning from 0.01 to 100 micromoles per liter, which covers the full physiological range and then some.26Nature Communications. A flexible and physically transient electrochemical sensor for real-time wireless nitric oxide monitoring The sensor dissolves harmlessly after a set period, eliminating the need for surgical removal. These devices remain in the proof-of-concept stage and are nowhere near routine clinical use, but they hint at a future where continuous, real-time NO monitoring could be as straightforward as a glucose monitor is for someone with diabetes.
For now, the practical reality is that a nitric oxide blood test remains a specialized, somewhat finicky investigation. It is most useful when ordered by a clinician who understands the confounders, interprets the result in context, and uses it as one piece of a larger cardiovascular or metabolic workup rather than a standalone diagnostic. The science behind it is solid, but the measurement itself requires more patient preparation and more interpretive nuance than most routine blood tests. If your doctor has ordered one, following the dietary and behavioral prep instructions closely is the single best thing you can do to ensure the result means something.