Acid-Base & ABGs — Read the Blood

Quick — a patient's pH is 7.28. Is that acidosis or alkalosis?

The Three Numbers That Matter

FOUNDATION

Every ABG gives you three critical values. Everything else is derived from these:

pH — the verdict. Normal: 7.35 – 7.45. Below 7.35 = acidosis. Above 7.45 = alkalosis. 🔑 7.4 is home base. Think of it like a 4-letter word: SAFE.

pCO2 — the respiratory player. Normal: 35–45 mmHg. CO2 is an acid. More CO2 = more acid. Lungs blow it off in minutes.

HCO3 — the metabolic player. Normal: 22–26 mEq/L. Bicarb is a base. Kidneys adjust it over hours to days.

The golden rule: CO2 and pH move in opposite directions. HCO3 and pH move in the same direction. That's it. That's the whole game. 🔑 CO2 is a contrarian — pH goes down, CO2 goes up. Bicarb is a yes-man — pH goes down, bicarb goes down too.

The 5-Step ABG Algorithm

THE METHOD

Every ABG. Same five steps. Same order. Don't skip. Don't freestyle.

Look at the pH.
< 7.35 = acidemia. > 7.45 = alkalemia. Normal pH with abnormal CO2/bicarb = compensated or mixed.

↓

Is it respiratory or metabolic?
Check which value explains the pH. If pH is low and pCO2 is high → respiratory acidosis. If pH is low and HCO3 is low → metabolic acidosis.

↓

Is there compensation?
The body always tries to compensate. Respiratory problem → kidneys adjust bicarb. Metabolic problem → lungs adjust CO2. Compensation moves toward normal but never overshoots.

↓

If metabolic acidosis: calculate the anion gap.
AG = Na − (Cl + HCO3). Normal = 12 (±2). Elevated = something is adding acid. Normal gap = losing bicarb.

↓

If anion gap is elevated: check the delta-deltaThe delta-delta (Δ/Δ) compares: how much the AG rose vs. how much HCO3 fell. If AG rose MORE than bicarb fell, there's a hidden metabolic alkalosis underneath. If AG rose LESS, there's an additional non-gap acidosis hiding..
ΔAG / ΔHCO3. Ratio <1 = additional non-gap acidosis. Ratio >2 = hidden metabolic alkalosis. Ratio 1–2 = pure gap acidosis.

Compensation NEVER overshoots. If pH goes past normal in the "compensating" direction, you have a mixed disorder, not overcompensation. Boards love this.

Walk Through It

PRACTICE

ABG comes back: pH 7.30, pCO2 55, HCO3 26. Walk the algorithm.

Step 1 — pH is 7.30. What's the verdict?

Step 2 — pH is low (acidemia). pCO2 is 55 (high). HCO3 is 26 (normal). What's the primary disorder?

Step 3 — HCO3 is 26 (normal). For acute respiratory acidosis, expected HCO3 rise is 1 per 10 rise in pCO2. pCO2 rose by 15. Is this acute or chronic?

The Anion Gap

HIGH YIELD

When you see metabolic acidosis (low pH + low bicarb), the next question is always: is the gap elevated?

Anion Gap = Na − (Cl + HCO3). Normal ≈ 12.

Elevated gap = something is adding acid to the blood. The unmeasured anions are the new acid.

Normal gap = you're losing bicarb (diarrhea, RTA) or gaining chloride. Cl replaces HCO3 so the gap stays normal.

MUDPILES — causes of anion gap metabolic acidosis:
Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates

HARDASS — causes of non-gap (hyperchloremic) metabolic acidosis:
Hyperalimentation, Acetazolamide, RTA, Diarrhea, Adrenal insufficiency, Saline infusion, Spironolactone

Albumin affects the gap. Every 1 g/dL drop in albumin below 4 lowers the AG by ~2.5. A "normal" AG of 12 in a patient with albumin of 2 is actually an elevated corrected AG of 17. Always correct for albumin in sick patients.

Elimination Game

INTERACTIVE

A patient presents obtunded. ABG: pH 7.22, pCO2 20, HCO3 8, Na 140, Cl 100. Six possible diagnoses. Each clue eliminates one. Find the answer.

COPD exacerbation

Diabetic ketoacidosis

Severe diarrhea

Renal tubular acidosis

Ethylene glycol ingestion

Lactic acidosis

CLUE 1

pCO2 is 20 (low). The lungs are hyperventilating, compensating for a metabolic problem. This is NOT a respiratory acidosis.

Eliminated: COPD exacerbation — COPD causes high CO2, not low.

CLUE 2

Anion gap = 140 − (100 + 8) = 32. Massively elevated. This is a gap acidosis.

Eliminated: Severe diarrhea & RTA — both cause non-gap acidosis (normal AG).

CLUE 3

Glucose is 95. No ketonuria. This is not a diabetic crisis.

Eliminated: DKA — no hyperglycemia, no ketones.

CLUE 4

Lactate is 1.8 (normal). The tissue oxygenation is fine.

Eliminated: Lactic acidosis — lactate is normal.

FINAL CLUE

Osmolar gap is elevated. Calcium oxalate crystals in the urine. The patient drank something they shouldn't have.

Answer: Ethylene glycol ingestion — gap acidosis + osmolar gap + oxalate crystals = toxic alcohol.

Compensation Rules

REFERENCE

You don't need to memorize the exact formulas. You need to know the direction and the magnitude.

Disorder	Primary Change	Compensation	Rule of Thumb
Metabolic acidosis	↓ HCO3	↓ pCO2 (hyperventilate)	Winter's: pCO2 = 1.5(HCO3) + 8 ±2
Metabolic alkalosis	↑ HCO3	↑ pCO2 (hypoventilate)	pCO2 rises ~0.7 per 1 HCO3 rise
Acute resp acidosis	↑ pCO2	↑ HCO3 (buffering)	HCO3 rises 1 per 10 pCO2 rise
Chronic resp acidosis	↑ pCO2	↑ HCO3 (renal)	HCO3 rises 3.5 per 10 pCO2 rise
Acute resp alkalosis	↓ pCO2	↓ HCO3	HCO3 falls 2 per 10 pCO2 drop
Chronic resp alkalosis	↓ pCO2	↓ HCO3 (renal)	HCO3 falls 5 per 10 pCO2 drop

The body compensates metabolic problems fast (lungs adjust in minutes). It compensates respiratory problems slow (kidneys take hours to days). That's how you tell acute from chronic.

ABG Interpreter

TOOL

Plug in the numbers. Watch the algorithm think out loud. Then try to beat it.

Enter ABG Values

pCO2 (mmHg)

HCO3 (mEq/L)

Na (mEq/L)

Cl (mEq/L)

Don't Kill Them

QUIZ

Four patients just rolled in with ABGs. Figure out what's wrong before something bad happens. No pressure.