Physiology · Renal, fluids and acid–base

Mixed acid–base disorders !

Measurement
Exam priority: medium

Sometimes two or three acid–base problems happen at once; they show up because the compensation doesn’t fit what is expected.

Updated 2026-11-01

Also called: Double and triple disorders, Inappropriate compensation

What it is

A mixed disorder is two or more primary disorders at the same time. Physiology provides the clues: compensation takes time and has limits, never overshoots normal pH and almost never fully normalises it. If PaCO₂ or HCO₃⁻ falls outside the expected compensation, another process is present. This page covers why mixed disorders happen and how to spot them; the steps and formulas are in Biochemistry.

Key facts

Respiratory compensation starts in minutes and is complete in 12–24 hours; renal compensation takes 3–5 days.
So the same high PaCO₂ comes with a different HCO₃⁻ in the first hours than after several days: duration changes what is expected.
Chronic respiratory alkalosis comes closest to a normal pH (for example in pregnancy or at altitude).
In Bogotá and other high cities baseline PaCO₂ is lower: comparing with local values avoids diagnosing a mixed disorder that isn’t there.

Combinations that get asked

Situation	Disorders	Why
Salicylate poisoning (adult)	Respiratory alkalosis + high anion gap acidosis	Direct respiratory-centre stimulation and metabolic uncoupling with organic acids
Sepsis	Respiratory alkalosis + lactic acidosis	Mediators and fever drive ventilation; hypoperfusion makes lactate
Cardiorespiratory arrest	Respiratory acidosis + lactic acidosis	No ventilation and no perfusion
Ketoacidosis with vomiting	High anion gap acidosis + metabolic alkalosis	Keto-acids on one side, HCl loss on the other
COPD on a diuretic	Respiratory acidosis + metabolic alkalosis	CO₂ retention plus volume contraction and hypokalaemia
Advanced cirrhosis with vomiting or diuretics	Respiratory alkalosis + metabolic alkalosis	Hyperventilation from toxins and progesterone, plus acid loss

Clinical notes

A patient with ketoacidosis and a normal PaCO₂ is not “well compensated”: respiratory compensation is missing (fatigue, sedation, pneumonia) and the patient is at risk.
In COPD with chronic hypercapnia, a raised HCO₃⁻ is expected; if it climbs much higher, a metabolic alkalosis has been added (diuretics, vomiting).

Test yourself

Think of the answer, then tap the question to see it.

A patient has pH 7.40, PaCO₂ 25 mmHg and HCO₃⁻ 15 mmol/L. Why is this not a simple compensated disorder?

Compensation almost never fully normalises pH. Both values badly deranged with an exactly normal pH point to two primary disorders (metabolic acidosis plus respiratory alkalosis), for example from salicylates or sepsis.

Why can respiratory acidosis and respiratory alkalosis not coexist?

PaCO₂ has a single value at any moment: high, low or normal. HCO₃⁻, by contrast, can receive an acid load and an H⁺ loss at the same time.

In ketoacidosis the anion gap rises by 18 mmol/L but HCO₃⁻ falls by only 6. What does that indicate?

There was more HCO₃⁻ than expected: alongside the high anion gap acidosis there is a metabolic alkalosis (for example from vomiting).

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In Biochemistry

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 26.5 Disorders of Acid-Base Balance (OpenStax (Rice University), 2022); Bacterial Sepsis (StatPearls (NCBI Bookshelf), 2024); Respiratory alkalosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 31). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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