Skip to content

Add your earlier progress to your account?

From before you signed in, this browser has:

Which plan do you want to keep?

You answered the plan questions before you signed in, and those answers don't match the plan saved on your account.

SettingSaved planNew answers

Study progress is waiting to be saved

The primary acid-base disorder

12 min

Skills you'll build2 skills
  • Tell acidemia from acidosis and alkalemia from alkalosis
  • Identify the primary acid-base process from pH, pCO₂, and bicarbonate

At the bench: Interpret a blood gas result with its specimen conditions

Try first

Try first

An arterial gas gives pH 7.27, pCO2 60 mm Hg and bicarbonate 27 mmol/L. Which primary process explains the pH?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Acidemia, alkalemia, acidosis and alkalosis

The pH describes the state of the blood. A pH below 7.35 is acidemia, and a pH above 7.45 is alkalemia.1 Acidosis and alkalosis describe the processes that add or remove acid. A patient can have an acidosis and an alkalosis at once, and the two can leave the pH close to 7.40. A pH within the reference interval can therefore mask a disorder.1

Metabolic and respiratory components

Bicarbonate is the metabolic component, and the kidneys control it. pCO2 is the respiratory component, and ventilation controls it. The pH follows the ratio of bicarbonate to dissolved CO2.1 A rise in bicarbonate raises pH, and a rise in pCO2 lowers it.

DisorderpHPrimary changeCompensating change
Metabolic acidosisLowBicarbonate fallspCO2 falls
Metabolic alkalosisHighBicarbonate risespCO2 rises
Respiratory acidosisLowpCO2 risesBicarbonate rises
Respiratory alkalosisHighpCO2 fallsBicarbonate falls

Identifying the primary process

Work in a fixed order:1

  1. Classify the pH as acidemia or alkalemia.
  2. Find the variable that moves pH in that direction. In acidemia, look for a low bicarbonate or a high pCO2. In alkalemia, look for a high bicarbonate or a low pCO2.
  3. That variable identifies the primary process.

The other variable usually moves the same way as the primary one. That shift is compensation. The lungs change pCO2 within minutes. The kidneys change bicarbonate over hours to days.1 Compensation brings pH back toward 7.40 and rarely reaches it. A compensating change is expected, so it is not called a second disorder at this stage. The next step tests whether its size fits the expected response.

Before any of this, the laboratory confirms the specimen: arterial or venous source, inspired oxygen, and a pH that fits the pCO2 and bicarbonate reported with it.1

Follow the bench demonstration

Sources for this step
References
  1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.

Watch one

Read each decision and its explanation.

An arterial gas from an adult gives these results.

Which primary process explains the pH?

TestResultReference intervalFlag
pH7.25 Low7.35–7.457.35–7.45Low
pCO228 mm Hg Low35–45 mm Hg35–45Low
Bicarbonate12 mmol/L Low22–28 mmol/L22–28Low

Specimen: Not measured on this analyzer. Arterial, room air

  1. Read the pH: 7.25 is below 7.35, so the blood shows acidemia.

    The pH sets the direction against which the other values are interpreted.

  2. Read the bicarbonate: 12 mmol/L is low, which moves pH down.

    A low bicarbonate removes base and lowers pH, so it can explain acidemia.

  3. Read the pCO2: 28 mm Hg is low, which moves pH up.

    A low pCO₂ removes acid and raises pH, so it partly offsets the acidemia.

  4. Identify it. The low bicarbonate explains the acidemia, so the primary process is metabolic acidosis.

    The variable that moves pH in the direction observed is the primary process.

  5. Interpret the pCO2. Its fall is in the expected direction of compensation, and its size is checked with the Winters formula in the next step.

    Ventilation lowers pCO₂ within minutes to offset a metabolic acidosis.

Acidemia from a primary metabolic acidosis, with pCO2 falling in the direction of compensation.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

An arterial gas gives pH 7.31, pCO₂ 30 mm Hg, and HCO₃⁻ 15 mmol/L. Which primary process explains the acidemia?

Incorrect. Respiratory acidosis requires a primary pCO₂ rise. Here pCO₂ is low, which opposes the acidemia.

Correct. The low bicarbonate explains the low pH. The lower pCO₂ moves pH toward normal. Assess expected compensation as a separate step.

Incorrect. A primary bicarbonate rise produces metabolic alkalosis. Bicarbonate is decreased here.

Hint
  1. Start with the pH and classify it.
  2. Ask which of the two variables pushes pH in that direction.

Review Base excess and base deficit

Problem 2 of 3 · Hints available

A patient has pH 7.40, a low bicarbonate from lactic acid and a low pCO2 from a separate cause of fast breathing. Which statement is correct?

A pH of 7.40 rules out acidemia. The lactic acid is still an acidosis, and the separate fall in pCO2 offsets it.

Used acidemia and acidosis as the same term

Acidemia names a measured pH below 7.35. Acidosis names a process that adds acid or removes base. An acidosis can coexist with an opposing process that keeps pH near 7.40, so a pH near 7.40 does not rule out an acidosis.

Acidemia means a pH below 7.35, and this pH is 7.40. Acidosis is a process that adds acid, and lactic acid production is one.

Acidemia is a measured pH below 7.35. With a pH of 7.40, an acidosis is present and acidemia is absent.

Used acidemia and acidosis as the same term

Acidemia names a measured pH below 7.35. Acidosis names a process that adds acid or removes base. An acidosis can coexist with an opposing process that keeps pH near 7.40, so a pH near 7.40 does not rule out an acidosis.

Hint
  1. Acidemia and acidosis describe different things.
  2. One describes the measured pH, and the other describes a process in the body.

Review The four primary acid-base disorders

Problem 3 of 3 · On your own

An arterial gas gives pH 7.49, pCO2 48 mm Hg and bicarbonate 36 mmol/L. Which primary process explains the pH?

The pH is high, and the raised bicarbonate is the value that pushes it up. The higher pCO2 partly offsets the alkalemia.

A primary respiratory acidosis lowers pH. This pH is 7.49, above the reference interval.

Breathing slows to raise pCO2 when bicarbonate rises. A pCO2 of 48 mm Hg is that expected response, and it is compared with the compensation rule before any second disorder is named.

Treated the compensating change as a second primary process

In a primary metabolic acidosis the pCO₂ falls as compensation, and in a primary respiratory disorder bicarbonate shifts. Calling the compensating change a second primary disorder reports a mixed disorder the results do not support. Compare the other variable with its expected compensation first.

A respiratory alkalosis starts with a low pCO2. Here pCO2 is raised.

Review The four primary acid-base disorders

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Deshawn Faber, 52, MRN 5520917, has had 3 days of vomiting.
  • An arterial gas is drawn on room air, with the source recorded on the requisition.
  • The syringe has no air and a correct fill.
  • QC for the run is acceptable.
TestResultReference intervalFlag
pH7.51 High7.35–7.457.35–7.45High
pCO247 mm Hg High35–45 mm Hg35–45High
Bicarbonate37 mmol/L High22–28 mmol/L22–28High

Specimen: Not measured on this analyzer. Arterial, room air; no air in the syringe

Decision 1 of 3

What state does the pH show?

Alkalosis describes a process. The pH describes the state of the blood, and a pH above 7.45 is alkalemia.

Used acidemia and acidosis as the same term

Acidemia names a measured pH below 7.35. Acidosis names a process that adds acid or removes base. An acidosis can coexist with an opposing process that keeps pH near 7.40, so a pH near 7.40 does not rule out an acidosis.

A pH of 7.51 is above 7.45, so the blood shows alkalemia.

7.51 is above the reference interval of 7.35 to 7.45.

Review The four primary acid-base disorders

Decision 2 of 3

Which primary process explains the pH?

A primary rise in pCO2 lowers pH. This pH is high.

A respiratory alkalosis starts with a low pCO2, and this pCO2 is raised.

The high bicarbonate pushes pH up and explains the alkalemia. Loss of stomach acid in vomiting fits it.

Review The four primary acid-base disorders

Decision 3 of 3

The pCO2 of 47 mm Hg is above the reference interval. How do you interpret it?

A rising bicarbonate slows breathing, and pCO2 rises in response. At a bicarbonate of 37 mmol/L the expected pCO2 is about 49 mm Hg, and 47 mm Hg fits it.

Treated the compensating change as a second primary process

In a primary metabolic acidosis the pCO₂ falls as compensation, and in a primary respiratory disorder bicarbonate shifts. Calling the compensating change a second primary disorder reports a mixed disorder the results do not support. Compare the other variable with its expected compensation first.

pCO2 rises about 0.7 mm Hg for each 1 mmol/L rise in bicarbonate above 24. That predicts about 49 mm Hg, close to the measured 47 mm Hg.

Air lowers pCO2. This pCO2 is raised, and the syringe had no air.

Review The four primary acid-base disorders

The pH is high, and only the bicarbonate moves pH in that direction. The raised pCO2 is in the direction compensation predicts, so the gas shows one primary process, a metabolic alkalosis.

For reference

Sources checked