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Blood Test Tracker

Arterial Blood pH

How acidic or basic your arterial blood is. The body holds it in a very narrow range, so small shifts matter.

AI-verified against cited sources. Awaiting expert reviewAlso called pHaLOINC 2744-1

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What it is

Blood pH tells you how acidic or basic your blood is. On the pH scale, 7 is neutral, and healthy arterial blood sits just on the basic side of that. Your lungs and kidneys work together to keep it steady. The lungs change how much carbon dioxide you breathe out, and the kidneys adjust bicarbonate. pH is measured on an arterial blood gas, a sample taken from an artery, usually at the wrist.

Why it matters

Your cells and enzymes work best within a narrow pH window. When pH moves outside the usual range, it can be a sign that breathing, the kidneys or the body's chemistry are under strain. The pH alone doesn't say why it changed, so it is read together with pCO2 and bicarbonate. A pH in the usual range doesn't always mean everything is in balance, because one change in the body can offset another.

If it’s low

A pH below the usual range is called acidemia. It means the blood is more acidic than usual. This can happen when carbon dioxide builds up because breathing isn't clearing it, or when the body makes too much acid or loses too much bicarbonate. Your clinician looks at pCO2 and bicarbonate next to see which one is at work.

Common causes

  • A sample that waited too long before testing, or was not purely arterial blood
  • Slow or shallow breathing, including from sedating medicines or opioids
  • Lung conditions that make it hard to breathe out carbon dioxide, such as COPD or pneumonia
  • Severe diarrhea, which causes bicarbonate loss
  • Kidney problems, since the kidneys clear acid from the body
  • Diabetic ketoacidosis, or a build-up of lactic acid when tissues are short of oxygen
  • Poisoning or an overdose

What can help

  • Ask your clinician what is driving the change, since the next steps depend on the cause
  • If you have diabetes or a lung or kidney condition, ask which warning signs should make you call right away
  • Ask whether a repeat sample is needed

If it’s high

A pH above the usual range is called alkalemia. It means the blood is more basic than usual. This can happen when you breathe off more carbon dioxide than normal, or when the body has gained base or lost acid. Many causes are short-lived and harmless. Your clinician looks at pCO2 and bicarbonate to sort out which kind it is.

Common causes

  • Fast breathing from anxiety, pain or the stress of the blood draw itself
  • Pregnancy, when breathing runs a little faster
  • Recent arrival at high altitude
  • Low oxygen levels or lung conditions that make you breathe faster
  • Vomiting, which removes stomach acid
  • Some water pills (diuretics) and other medicines that change fluid and salt balance
  • Hormone conditions such as too much aldosterone

What can help

  • Ask your clinician whether the cause is related to your breathing or to your body's chemistry
  • Review any water pills or other medicines with your clinician before changing anything
  • Ask whether a repeat sample or other tests, such as electrolytes, would help

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 7.35–7.45 pH. Source: MedlinePlus, National Library of Medicine, South African Family Practice, Medicine International, Penn State Health, Northwell Health Laboratories. Labs treat below 7.2 pH or above 7.6 pH as urgent.

What can affect your result

  • Sample type: venous blood runs a little lower than arterial blood
  • How the sample was handled: air bubbles, too much liquid heparin, or a delay before testing
  • Your breathing at the moment of the draw, including pain, anxiety or breath-holding
  • Pregnancy, when breathing runs a little faster
  • Temperature: most analyzers report pH at 37 °C, not at your actual body temperature
  • Air bubbles in the syringe, too much liquid heparin, or a delay before analysis can shift pH (and pCO2 and pO2). Putting the sample on ice and testing it promptly limits this.
  • A venous or mixed sample mislabeled as arterial reads lower in pH.
  • Pain, anxiety or holding your breath during the needle stick can change your breathing and move the pH.
  • A recent change in oxygen or ventilator settings can change the result, so the timing of the draw matters.

A change in breathing moves pH within minutes. The kidneys take longer, from hours to days, to adjust bicarbonate.

When to talk to a clinician

An arterial blood gas is usually done in a clinic or hospital, where your care team sees the result right away. Many US hospitals treat a pH below about 7.20 or above about 7.60 as needing prompt attention and call the ordering clinician. If you are not in a care setting and feel very short of breath, confused, unusually drowsy or faint, or have chest pain, call 911 or your local emergency number.

Questions you could ask

  • Was this sample taken from an artery, and was I on extra oxygen at the time?
  • How do my pH, pCO2 and bicarbonate fit together?
  • Is my high pH related to my breathing or to my blood chemistry?
  • Could pain, anxiety or the way I was breathing during the draw have affected this result?
  • Is this more likely related to my lungs or to my metabolism?
  • Should this be repeated, and are other tests, such as bicarbonate or lactate, worth checking?

Questions people ask

Is blood pH the same as urine pH?

No. Blood pH is held in a very narrow range by your lungs and kidneys. Urine pH varies much more, depends partly on what you eat, and is a separate test.

Can food or 'alkaline' water change my blood pH?

In healthy people, no. The lungs and kidneys correct small shifts, so everyday food and drink do not move blood pH out of its usual range. Diet can change urine pH, which is a different measurement.

My report shows hydrogen ion (H+) in nmol/L instead of pH. How do I compare?

Some labs, mostly outside the US, print the hydrogen ion level instead. A pH of 7.40 is about 40 nmol/L. The usual range of 7.35 to 7.45 is about 35 to 45 nmol/L. Lower pH means a higher H+ number.

Show the science and sources

For clinicians and researchers

Arterial pH follows the Henderson-Hasselbalch relationship: pH = 6.1 + log10([HCO3-] / (0.03 x pCO2)), with HCO3- in mmol/L and pCO2 in mmHg. Analyzers measure pH and pCO2 directly at 37 °C and calculate bicarbonate, so the three values should agree. If a pH recalculated from the reported HCO3- and pCO2 differs from the reported pH by more than about 0.05, an error in one value is suspected. Normal [H+] is about 35-45 nmol/L. Because pH is logarithmic, a change of 0.3 units doubles or halves [H+], and [H+] in nmol/L equals 10^(9 - pH). Air bubbles, excess liquid heparin and delayed analysis distort results; one review advises analyzing room-temperature samples within 15 minutes and iced samples within 1 hour, in line with the 1992 AARC sampling guideline. Venous pH runs roughly 0.02-0.03 below arterial. A normal pH does not exclude an acid-base disorder, because compensation or a mixed disorder can bring pH back toward normal. The 7.20 and 7.60 limits are medians of hospital notification lists, not a clinical standard.

LOINC 2744-1 (primary), 11558-4, 2745-8. Specimen: whole-blood.

Sources

  1. Arterial Blood Gas (ABG) Test, MedlinePlus, National Library of Medicine
  2. Mastering blood gas interpretation: A practical guide for primary care providers, South African Family Practice (2025)
  3. Blood gas analysis: Clinical applications, interpretation and future directions (Review), Medicine International (2026)
  4. Visualization of Critical Limits and Critical Values Facilitates Interpretation, Diagnostics (MDPI) (2025)
  5. Critical Decision Thresholds for Urgent Physician Notification of Point-of-Care Testing Results, Diagnostics (MDPI) (2026)
  6. Blood Gas, Arterial, Penn State Health
  7. Blood Gas Arterial, Northwell Health Laboratories

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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