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Summary Table

Feature	Detail
Risk Factors	Varies by disorder: COPD/sedative overdose (respiratory acidosis), hyperventilation/anxiety/PE (respiratory alkalosis), DKA/renal failure/toxins (metabolic acidosis), vomiting/diuretics (metabolic alkalosis)
Etiology	Imbalance between acid production/excretion and buffer systems involving $CO_2$ (respiratory component) and $HCO_3^-$ (metabolic component)
Patient Presentation	Altered mental status, Kussmaul breathing, tetany, arrhythmias, confusion, depending on the type and severity
Classic Physical Exam	Kussmaul breathing (metabolic acidosis), shallow breathing (metabolic alkalosis), carpopedal spasm/Chvostek sign (respiratory alkalosis), narcosis/somnolence (respiratory acidosis)
Key Diagnostic Results	Arterial blood gas (ABG): pH, $PaCO_2$, $HCO_3^-$; serum electrolytes for anion gap; urine electrolytes when needed
Management	Treat the underlying cause first; bicarbonate replacement only in severe metabolic acidosis ($pH < 7.1$); acetazolamide for metabolic alkalosis; ventilatory support for respiratory disorders
Keywords	Kussmaul breathing, anion gap, Winter's formula, delta-delta, compensation, MUDPILES, HARDUPS, contraction alkalosis, saline-responsive vs. saline-resistant

0. Foundation

Before diving into individual disorders, you need a bulletproof algorithm. Every exam question on acid-base gives you an ABG and expects you to identify: (1) the primary disorder, (2) whether compensation is present, and (3) whether compensation is appropriate or a second disorder is hiding.

Normal Values You Must Memorize

Parameter	Normal Value
pH	7.35 to 7.45
$PaCO_2$	35 to 45 mmHg
$HCO_3^-$	22 to 26 mEq/L

The 5-Step ABG Interpretation Algorithm

Step 1: Look at the pH. If $pH < 7.35$, the primary process is an acidosis. If $pH > 7.45$, the primary process is an alkalosis. If the pH is normal (7.35 to 7.45), look at the other values because a mixed disorder or full compensation may be present.

Step 2: Identify the primary disorder. Match the pH direction with the deranged component. Ask: "Which value explains the pH?"

pH direction	If $PaCO_2$ is the culprit	If $HCO_3^-$ is the culprit
Acidemia ($pH < 7.35$)	$PaCO_2 > 45$ = Respiratory Acidosis	$HCO_3^- < 22$ = Metabolic Acidosis
Alkalemia ($pH > 7.45$)	$PaCO_2 < 35$ = Respiratory Alkalosis	$HCO_3^- > 26$ = Metabolic Alkalosis

Step 3: Check for compensation. The body never overcompensates. The compensatory response always moves in the same direction as the primary change. This is a fundamental rule that exam writers exploit.

Step 4: If metabolic acidosis is present, calculate the anion gap.

Step 5: If an elevated anion gap is found, calculate the delta-delta ratio to check for a hidden second metabolic disorder.

1. Pathophysiology

The body maintains blood pH within a narrow range of 7.35 to 7.45 through three integrated systems: chemical buffers (bicarbonate being the most important), respiratory regulation of $CO_2$, and renal regulation of $HCO_3^-$.

The relationship is captured by the Henderson-Hasselbalch equation:

$$pH = 6.1 + \log \frac{[HCO_3^-]}{0.03 \times PaCO_2}$$

This equation is the single most important concept in acid-base physiology. It tells you that pH depends on the ratio of bicarbonate to carbon dioxide, not their absolute values. This is why compensation works: if one side changes, the other side shifts in the same direction to preserve the ratio and bring pH back toward normal.

Respiratory disorders arise from changes in $PaCO_2$. Carbon dioxide is a volatile acid. When ventilation decreases, $CO_2$ accumulates and pH drops (respiratory acidosis). When ventilation increases, $CO_2$ is blown off and pH rises (respiratory alkalosis). The kidneys compensate over days by adjusting $HCO_3^-$ reabsorption.

Metabolic disorders arise from changes in $HCO_3^-$. When bicarbonate is consumed (by acid loads) or lost (through the GI tract or kidneys), pH drops (metabolic acidosis). When bicarbonate accumulates (from vomiting, diuretics, or excess alkali), pH rises (metabolic alkalosis). The lungs compensate within minutes to hours by adjusting ventilation.

A critical teaching point: respiratory compensation is fast (minutes to hours) but limited, while renal compensation is slow (3 to 5 days) but more powerful. This is why you must distinguish acute from chronic respiratory disorders; the expected compensation values are different.

2. Classification and Clinical Manifestation

The Four Primary Disorders

Disorder	pH	Primary Change	Compensation	Clinical Setting
Respiratory Acidosis	Low	$PaCO_2$ elevated	$HCO_3^-$ rises	COPD exacerbation, opioid overdose, neuromuscular disease, obesity hypoventilation
Respiratory Alkalosis	High	$PaCO_2$ decreased	$HCO_3^-$ falls	Anxiety/panic, early PE, early sepsis, salicylate toxicity (early), pregnancy, high altitude, liver cirrhosis
Metabolic Acidosis	Low	$HCO_3^-$ decreased	$PaCO_2$ falls	DKA, lactic acidosis, renal failure, diarrhea, RTA, toxic ingestions
Metabolic Alkalosis	High	$HCO_3^-$ elevated	$PaCO_2$ rises	Vomiting/NG suction, diuretic use, hyperaldosteronism, Cushing syndrome

Clinical Manifestations by System

System	Acidosis (Respiratory or Metabolic)	Alkalosis (Respiratory or Metabolic)
Neurological	Confusion, somnolence, coma (especially respiratory acidosis with $CO_2$ narcosis)	Light-headedness, confusion, seizures (severe alkalosis)
Respiratory	Kussmaul breathing (deep, rapid) in metabolic acidosis; shallow/slow breathing in respiratory acidosis	Hyperventilation in respiratory alkalosis; hypoventilation (compensatory) in metabolic alkalosis
Cardiovascular	Decreased myocardial contractility, vasodilation, predisposition to arrhythmias; hyperkalemia shifts	Arrhythmias (especially with concurrent hypokalemia); hypokalemia shifts
Neuromuscular	Hyperkalemia-related weakness	Tetany, carpopedal spasm, perioral tingling (from decreased ionized calcium in alkalosis)

The connection between acid-base status and potassium is heavily tested. In acidosis, $H^+$ enters cells and $K^+$ exits, causing hyperkalemia. In alkalosis, the reverse occurs, causing hypokalemia. This is why DKA patients present with high serum potassium despite having total body potassium depletion; once insulin and fluids correct the acidosis, potassium plummets.

3. Compensation

This section is the backbone of every exam question on acid-base disorders. You are not simply being tested on whether you can identify a disorder. You are being tested on whether you can determine if the compensation is appropriate. If the measured compensatory value does not match the predicted value, a second (hidden) disorder exists.

Definitions

Status	Definition	pH	What It Means
Uncompensated	Only the primary value is abnormal; the compensatory parameter has not yet changed	Clearly abnormal	The disorder is very acute (minutes old) or the compensatory organ has failed
Partially Compensated	Both the primary value and the compensatory value are abnormal, but pH remains outside normal range	Abnormal, but closer to normal than uncompensated	The body is responding but has not had enough time or capacity to normalize pH
Fully Compensated	Both the primary value and compensatory value are abnormal, but pH has returned to the normal range (7.35 to 7.45)	Normal range (but look at which side of 7.40 it falls on)	Enough time has passed for the compensatory system to bring pH back

When pH is normal but both $PaCO_2$ and $HCO_3^-$ are abnormal, you have a fully compensated disorder. To determine the primary disorder, look at which side of 7.40 the pH falls. A pH of 7.37 (low-normal) suggests a compensated acidosis. A pH of 7.43 (high-normal) suggests a compensated alkalosis. The body never overcompensates.

The Compensation Formulas

Primary Disorder	Expected Compensation	Formula
Metabolic Acidosis	$PaCO_2$ drops	Winter's Formula: $PaCO_2 = 1.5 \times [HCO_3^-] + 8 \ (\pm 2)$
Metabolic Alkalosis	$PaCO_2$ rises	$PaCO_2 = 0.7 \times [HCO_3^-] + 21 \ (\pm 2)$
Acute Respiratory Acidosis	$HCO_3^-$ rises	For every $\uparrow 10 \ PaCO_2$, $HCO_3^-$ rises by 1
Chronic Respiratory Acidosis	$HCO_3^-$ rises	For every $\uparrow 10 \ PaCO_2$, $HCO_3^-$ rises by 3.5
Acute Respiratory Alkalosis	$HCO_3^-$ falls	For every $\downarrow 10 \ PaCO_2$, $HCO_3^-$ falls by 2
Chronic Respiratory Alkalosis	$HCO_3^-$ falls	For every $\downarrow 10 \ PaCO_2$, $HCO_3^-$ falls by 5

How to Apply These Formulas

Example: A patient has pH 7.30, $PaCO_2$ 30, $HCO_3^-$ 14.

Step 1: pH is low, so this is an acidosis. Step 2: $HCO_3^-$ is low (14), which explains the low pH. This is a metabolic acidosis. Step 3: The $PaCO_2$ is 30, which is below normal. This is expected respiratory compensation (hyperventilation). But is it appropriate?

Apply Winter's Formula: $$PaCO_2 = 1.5(14) + 8 = 29 \ (\pm 2)$$

The predicted range is 27 to 31. The measured $PaCO_2$ is 30. This falls within the predicted range, so compensation is appropriate. This is a simple metabolic acidosis with appropriate respiratory compensation.

If the measured $PaCO_2$ were 40 (higher than expected), the patient is not ventilating enough. This means a concurrent respiratory acidosis is also present.

If the measured $PaCO_2$ were 20 (lower than expected), the patient is ventilating too much. This means a concurrent respiratory alkalosis is also present.

Worked Example

Scenario	pH	$PaCO_2$	$HCO_3^-$	Interpretation
A	7.25	40	16	Metabolic acidosis, uncompensated (the $PaCO_2$ has not dropped yet)
B	7.30	30	14	Metabolic acidosis, partially compensated (the lungs are blowing off $CO_2$, but pH is still abnormal)
C	7.37	28	16	Metabolic acidosis, fully compensated (pH is back in normal range, sitting on the acidotic side of 7.40)
D	7.52	48	38	Metabolic alkalosis, partially compensated ($PaCO_2$ is rising but pH remains elevated)

4. Anion Gap

When you identify a metabolic acidosis, the very next step is always to calculate the anion gap. This is not optional. Every exam question expects you to do this.

$$Anion \ Gap = [Na^+] - [Cl^-] - [HCO_3^-]$$

Normal anion gap: 12 $(\pm 2)$ mEq/L (some references use 8 to 12 depending on whether albumin-corrected).

Correcting for Albumin

Because albumin is an unmeasured anion, hypoalbuminemia falsely lowers the anion gap. For every 1 g/dL decrease in albumin below 4.0, add 2.5 to the calculated AG.

$$Corrected \ AG = Calculated \ AG + 2.5 \times (4.0 - [Albumin])$$

This correction is tested and is clinically relevant in ICU patients, cirrhotics, and nephrotic syndrome patients.

High Anion Gap Metabolic Acidosis (HAGMA)

The classic mnemonic is MUDPILES (though variants like GOLDMARK exist):

Letter	Cause	Key Feature
M	Methanol	Osmolar gap elevated, visual disturbances, formate toxicity
U	Uremia (renal failure)	Elevated BUN/creatinine, chronic kidney disease
D	Diabetic ketoacidosis	Hyperglycemia, ketonuria/ketonemia, fruity breath
P	Propylene glycol / Paraldehyde	Iatrogenic (IV lorazepam drips), osmolar gap
I	Isoniazid / Iron toxicity	Seizures (isoniazid); GI hemorrhage then shock (iron)
L	Lactic acidosis	Tissue hypoperfusion, sepsis, shock, seizures, metformin
E	Ethylene glycol	Osmolar gap, calcium oxalate crystals in urine, renal failure
S	Salicylates	Tinnitus, mixed respiratory alkalosis AND HAGMA (very testable)

Normal Anion Gap Metabolic Acidosis (NAGMA)

The mnemonic is HARDUPS:

Letter	Cause	Key Feature
H	Hyperalimentation (TPN)	Amino acid metabolism generates $HCl$
A	Acetazolamide / Addison disease	Carbonic anhydrase inhibitors cause bicarb wasting
R	Renal tubular acidosis (RTA)	Types 1, 2, and 4 (see below)
D	Diarrhea	Loss of $HCO_3^-$-rich intestinal fluid; most common cause
U	Ureteral diversions	Sigmoid or ileal conduits reabsorb $Cl^-$ and secrete $HCO_3^-$
P	Pancreatic fistula	Loss of $HCO_3^-$-rich pancreatic secretions
S	Saline infusion (large volume)	Dilutional/hyperchloremic acidosis from $0.9%$ $NaCl$

A distinguishing feature of NAGMA is that it is always hyperchloremic. As $HCO_3^-$ is lost, $Cl^-$ rises to maintain electroneutrality, so the anion gap stays normal.

The Delta-Delta Ratio

Once you find an elevated anion gap, you must check whether the rise in the anion gap is proportional to the fall in bicarbonate. This is the delta-delta calculation.

$$\Delta AG = Calculated \ AG - 12$$ $$\Delta HCO_3^- = 24 - Measured \ HCO_3^-$$ $$Delta\text{-}Delta \ Ratio = \frac{\Delta AG}{\Delta HCO_3^-}$$

Ratio	Interpretation
< 1	The $HCO_3^-$ has fallen more than the AG has risen. There is a concurrent non-anion gap metabolic acidosis hiding alongside the HAGMA.
1 to 2	Pure anion gap metabolic acidosis. The changes are proportional.
> 2	The AG has risen more than the $HCO_3^-$ has fallen. There is a concurrent metabolic alkalosis that is keeping the $HCO_3^-$ artificially high.

This is the exam writer's favorite tool for testing advanced acid-base reasoning. A patient with DKA who is also vomiting will have a HAGMA with a delta-delta > 2, because the vomiting is generating a concurrent metabolic alkalosis that prevents the bicarbonate from dropping as much as expected.

The single most tested differentiator: Type 4 is the only RTA with hyperkalemia. If you see NAGMA plus hyperkalemia in a diabetic patient, the answer is Type 4 RTA.

5. Diagnostic Workup

Test	Purpose	When to Order
Arterial Blood Gas (ABG)	Determine pH, $PaCO_2$, $HCO_3^-$	First test in any suspected acid-base disorder
Basic Metabolic Panel (BMP)	Obtain $Na^+$, $K^+$, $Cl^-$, $HCO_3^-$, BUN, creatinine, glucose	Simultaneously with ABG
Anion Gap Calculation	Classify metabolic acidosis	Whenever metabolic acidosis is identified
Serum Albumin	Correct anion gap for hypoalbuminemia	Always in ICU, cirrhosis, nephrotic syndrome
Serum Lactate	Identify lactic acidosis	Sepsis, shock, altered mental status
Serum Ketones	Identify ketoacidosis	DKA, alcoholic ketoacidosis, starvation
Osmolar Gap	Screen for toxic alcohols	Suspected methanol or ethylene glycol ingestion
Urine Anion Gap	Differentiate renal vs. extra-renal cause of NAGMA	Normal anion gap metabolic acidosis of unclear etiology
Urine pH	Classify RTA	Suspected renal tubular acidosis

The Urine Anion Gap (A Critical Exam Tool)

$$Urine \ AG = [Na^+{urine}] + [K^+{urine}] - [Cl^-_{urine}]$$

This is used in NAGMA to determine whether the kidneys are appropriately excreting acid (as $NH_4^+$).

Result	Interpretation
Negative (e.g., -20)	Kidneys are working properly; extra-renal cause (diarrhea is the classic example). The $Cl^-$ is high because $NH_4Cl$ is being excreted.
Positive (e.g., +15)	Kidneys cannot excrete acid appropriately; renal cause (RTA).

The mnemonic: "Negative is Normal" for GI losses. If the urine anion gap is negative in NAGMA, think diarrhea. If positive, think RTA.

The Osmolar Gap

$$Osmolar \ Gap = Measured \ Osmolality - Calculated \ Osmolality$$

$$Calculated \ Osmolality = 2[Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$$

An osmolar gap > 10 is abnormal and suggests the presence of unmeasured osmoles, most commonly toxic alcohols (methanol, ethylene glycol, isopropanol) or propylene glycol. This is a screening test, not a confirmatory test.

6. Management and Treatment

Disorder	First-Line Treatment	Key Details
Metabolic Acidosis (HAGMA)	Treat the underlying cause	DKA: insulin + IV fluids + potassium; Toxic alcohols: fomepizole + dialysis; Lactic acidosis: restore perfusion
Metabolic Acidosis (severe, pH < 7.1)	IV sodium bicarbonate	Only when pH is dangerously low; goal is to raise pH above 7.1 to 7.2, not to normalize it
Metabolic Alkalosis (saline-responsive)	IV normal saline ($0.9% \ NaCl$)	Replenish volume and chloride; this is the treatment for vomiting-induced or diuretic-induced alkalosis
Metabolic Alkalosis (saline-resistant)	Treat the underlying hyperaldosteronism	Spironolactone for primary hyperaldosteronism; surgery for Conn syndrome or Cushing syndrome
Respiratory Acidosis (acute)	Improve ventilation	Non-invasive positive pressure ventilation (BiPAP) or intubation; naloxone if opioid-induced
Respiratory Acidosis (chronic)	Treat underlying lung disease	Bronchodilators, steroids for COPD; avoid excessive oxygen in chronic $CO_2$ retainers
Respiratory Alkalosis	Treat the underlying cause	Anxiolysis for panic attacks; treat PE, sepsis, or pain as appropriate

Metabolic Acidosis Management

For DKA, the management sequence is: (1) aggressive IV fluid resuscitation with normal saline, (2) potassium replacement if $K^+ < 5.3$ mEq/L before starting insulin (because insulin drives $K^+$ intracellularly and can cause fatal hypokalemia), (3) IV insulin drip at 0.1 units/kg/hour, (4) monitor glucose hourly and switch to dextrose-containing fluids when glucose reaches approximately 200 to 250 mg/dL to prevent hypoglycemia while continuing insulin to clear ketones. The anion gap, not the glucose, determines when to stop the insulin drip.

For toxic alcohol ingestion (methanol, ethylene glycol), the treatment is fomepizole (inhibits alcohol dehydrogenase, blocking conversion to toxic metabolites) plus hemodialysis for severe cases ($pH < 7.25$, renal failure, visual symptoms with methanol, or very high levels). Ethanol infusion is an alternative if fomepizole is unavailable.

Bicarbonate therapy is controversial and reserved for severe acidosis. The generally accepted indications include pH below 7.1, severe hyperkalemia with ECG changes, or certain toxic ingestions. The dose is typically 1 to 2 mEq/kg IV. Risks of bicarbonate include volume overload, overshoot alkalosis, hypokalemia (as $H^+$ shifts intracellularly and $K^+$ follows), and paradoxical CNS acidosis (because $CO_2$ crosses the blood-brain barrier more readily than bicarbonate).

Metabolic Alkalosis Management

The most important classification is saline-responsive vs. saline-resistant, determined by urine chloride.

Category	Urine $Cl^-$	Causes	Treatment
Saline-responsive	< 20 mEq/L	Vomiting, NG suction, diuretic use (past), post-hypercapnia	IV normal saline, KCl replacement
Saline-resistant	> 20 mEq/L	Primary hyperaldosteronism, Cushing syndrome, Bartter/Gitelman syndrome, current diuretic use	Treat underlying cause, spironolactone, amiloride

In vomiting-induced metabolic alkalosis, the mechanism is loss of $HCl$ from the stomach. The kidneys initially try to excrete the excess bicarbonate, but because of concurrent volume depletion and chloride depletion, the kidneys paradoxically reabsorb bicarbonate (contraction alkalosis). This is why saline works: it replenishes volume and chloride, allowing the kidneys to finally excrete the excess bicarbonate.

Key Management Point for Respiratory Acidosis

In chronic $CO_2$ retainers (e.g., severe COPD), the hypoxic drive becomes the primary stimulus for breathing because the central chemoreceptors have adapted to chronic hypercapnia. Giving high-flow oxygen can suppress this drive and worsen hypoventilation. The target oxygen saturation in these patients is 88 to 92%, not 95 to 100%.

7. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
DKA vs. Alcoholic Ketoacidosis (AKA)	Both are HAGMA with ketones	DKA: hyperglycemia, high glucose. AKA: low or normal glucose, history of binge drinking followed by poor oral intake
DKA vs. Starvation Ketosis	Both produce ketones	Starvation ketosis is mild; $HCO_3^-$ rarely drops below 18, and pH is usually normal
Metabolic Acidosis vs. Respiratory Acidosis	Both cause low pH	Look at the primary driver: low $HCO_3^-$ (metabolic) vs. high $PaCO_2$ (respiratory). Breathing pattern is the clinical clue: Kussmaul = metabolic; hypoventilation = respiratory
Metabolic Alkalosis (vomiting) vs. Respiratory Alkalosis (anxiety)	Both cause high pH	Vomiting: $HCO_3^-$ is high, $PaCO_2$ may be mildly elevated (compensatory). Anxiety: $PaCO_2$ is low, $HCO_3^-$ is normal or slightly low
Salicylate Toxicity vs. Simple HAGMA	Salicylates cause HAGMA	Salicylates produce a mixed pattern: early respiratory alkalosis (direct medullary stimulation) PLUS later HAGMA. This combination is the exam fingerprint. Tinnitus is the buzzword.
Type 1 RTA vs. Type 4 RTA	Both are NAGMA	Potassium is the differentiator. Type 1: hypokalemia + urine pH > 5.5. Type 4: hyperkalemia + urine pH < 5.5
Diarrhea vs. RTA	Both cause NAGMA	Use the urine anion gap. Diarrhea: negative UAG. RTA: positive UAG.
Contraction alkalosis vs. Primary hyperaldosteronism	Both cause metabolic alkalosis with hypokalemia	Urine $Cl^-$ separates them. Contraction: $Cl^- < 20$ (saline-responsive). Hyperaldosteronism: $Cl^- > 20$ (saline-resistant), with hypertension

8. Mixed Acid-Base Disorders

Mixed disorders are tested at the highest level. These occur when two or more primary disorders coexist simultaneously. Some classic combinations you must recognize:

Clinical Scenario	Expected Disorders	How You Detect It
Salicylate toxicity	Respiratory alkalosis + HAGMA	Low $PaCO_2$ beyond what compensation predicts; elevated anion gap
COPD patient with vomiting	Respiratory acidosis + metabolic alkalosis	pH may be near normal; $PaCO_2$ is high AND $HCO_3^-$ is very high (higher than expected for compensation alone)
DKA patient with vomiting	HAGMA + metabolic alkalosis	Delta-delta ratio > 2; the $HCO_3^-$ is higher than expected for the degree of anion gap elevation
Septic shock	Respiratory alkalosis + lactic (HAGMA) acidosis	Early sepsis drives hyperventilation (low $PaCO_2$) while poor tissue perfusion generates lactate
Cardiac arrest / respiratory failure + renal failure	Respiratory acidosis + metabolic acidosis	Both $PaCO_2$ and $HCO_3^-$ are moving in opposite directions (high $CO_2$, low bicarb); pH is profoundly low

The clue to a mixed disorder is that the compensation does not match what the formulas predict. When the measured compensatory value is outside the expected range, a second primary process is present.

9. Traps and High-Yield Pearls

The single most common way students fail acid-base questions is by stopping after identifying the primary disorder without checking compensation. The exam deliberately gives you ABGs where the numbers look like a simple disorder, but the compensation is off, hiding a second process. Always calculate.

Trap 1: Normal pH does not mean normal. A pH of 7.40 with a $PaCO_2$ of 60 and $HCO_3^-$ of 36 is not a "normal ABG." It is a fully compensated respiratory acidosis. If you see normal pH with abnormal $PaCO_2$ and $HCO_3^-$, that is pathology.

Trap 2: Forgetting to calculate the anion gap. Every time you identify metabolic acidosis, the anion gap calculation is mandatory. Skipping this step means you miss the etiology.

Trap 3: Forgetting the delta-delta. You found an elevated anion gap. Great. But is the bicarbonate dropping proportionally? If not, there is a hidden metabolic alkalosis or an additional NAGMA. This is the highest-yield calculation for advanced questions.

Trap 4: Confusing acute and chronic compensation in respiratory disorders. A patient with chronic COPD will have a much higher compensatory $HCO_3^-$ than an acute asthma attack patient with the same $PaCO_2$. Using the acute formula for a chronic patient will make you falsely diagnose a concurrent metabolic alkalosis.

Trap 5: Salicylate toxicity. This is the classic mixed disorder on exams. Students see the HAGMA and stop. They miss the concurrent respiratory alkalosis, which is the early and dominant feature. If a vignette mentions tinnitus, altered mental status, and an ABG showing both low $PaCO_2$ and elevated anion gap, salicylate poisoning is the answer.

Trap 6: Potassium in DKA. The serum potassium is high on presentation despite total body potassium depletion. Never give insulin until you confirm $K^+ \geq 3.3$ mEq/L (some guidelines use 5.3 as the threshold above which you hold potassium replacement, not insulin). Starting insulin with low potassium causes fatal arrhythmias.

The core competency being tested across all acid-base questions is your ability to systematically interpret ABGs using the 5-step algorithm, apply the compensation formulas to detect hidden disorders, and connect the laboratory findings to a clinical scenario that determines treatment. The exam rewards methodical, stepwise reasoning and punishes pattern-matching shortcuts.

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