Metabolic alkalosis workup

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Metabolic alkalosis: practical workup

1. Confirm the primary disorder

Order:

BMP: Na, K, Cl, HCO3-, BUN/creatinine, glucose
VBG or ABG: pH and PaCO2
Magnesium, phosphate, ionized calcium if moderate-severe or symptomatic
ECG if hypokalemia or severe alkalemia

Metabolic alkalosis is supported by elevated serum bicarbonate with alkalemia, unless a mixed disorder is present.

Expected respiratory compensation:

[ Expected\ PaCO_2 = 0.7 \times (HCO_3^- - 24) + 40 \pm 5 ]

A PaCO2 outside this range suggests an additional respiratory disorder. The Merck diagnostic overview also recommends blood gas and serum electrolyte assessment.

2. Focused history and examination

Ask specifically about:

Vomiting, nasogastric suction, gastric outlet obstruction
Diuretics, including recent or hidden use
Alkali exposure: bicarbonate, calcium carbonate, citrate-containing transfusions
Laxatives, diarrhea syndromes with chloride loss
Licorice, herbal products, glucocorticoids, mineralocorticoids
Hypertension, edema, volume depletion, polyuria
Family history of salt-wasting tubulopathy
Recent hypercapnia correction, especially after ventilation in COPD

Examine volume status and blood pressure. These two features become particularly useful after measuring urine chloride.

3. Obtain a spot urine chloride

This is the main branching test. Urine sodium is less useful because it may be elevated from bicarbonaturia.

Urine chloride	Interpretation	Common causes
<20 mmol/L	Chloride-responsive, usually effective-volume depleted	Vomiting, NG suction, remote diuretic use, chloride-wasting diarrhea, post-hypercapnic alkalosis
≥20 mmol/L	Chloride-resistant or ongoing renal chloride loss	Active diuretics, Bartter/Gitelman syndrome, severe K/Mg depletion, mineralocorticoid excess

A threshold around 20 mmol/L is standard. Values below this suggest renal chloride conservation and a saline-responsive process; higher values point to ongoing renal chloride loss or a mineralocorticoid-driven state. See the Merck approach and Cleveland Clinic review.

Important caveats

A patient actively taking a loop or thiazide may have a high urine chloride, even if volume depleted.
In advanced CKD, urine indices are less reliable.
Repeat urine chloride if the result conflicts with the clinical picture or diuretic timing is uncertain.

4. If urine chloride is low: identify extrarenal chloride loss

Check:

Vomiting/NG output and medication history
Urine potassium and serum K/Mg
Consider gastric obstruction if persistent vomiting or large-volume gastric losses
Consider recent resolution of chronic hypercapnia
Consider chloride-rich diarrhea, villous adenoma, or congenital chloride diarrhea when clinically plausible

Typical pattern: hypochloremia, hypokalemia, elevated bicarbonate, low urine chloride, orthostasis or low effective arterial volume.

5. If urine chloride is high: split by blood pressure

A. High urine chloride + hypertension

Think mineralocorticoid effect.

Order:

Plasma aldosterone and renin, ideally after correcting potassium and accounting for interfering antihypertensives when feasible
Serum cortisol or screening for Cushing syndrome if suggested clinically
Medication/substance review: licorice, steroids, fludrocortisone
Consider urine diuretic screen if surreptitious diuretic use is possible

Interpret renin/aldosterone broadly:

Renin	Aldosterone	Consider
Low	High	Primary aldosteronism
High	High	Renovascular hypertension, renin-secreting state, diuretics
Low	Low	Licorice/apparent mineralocorticoid excess, Liddle syndrome, exogenous mineralocorticoid, Cushing syndrome, congenital adrenal hyperplasia variants

For hypokalemic alkalosis with high urine chloride and hypertension, renin and aldosterone help organize the endocrine differential, as summarized in this CCJM diagnostic discussion.

B. High urine chloride + normal or low blood pressure

Think renal salt wasting or ongoing diuretic effect.

Order:

Urine diuretic assay when history is uncertain
Serum magnesium and urine calcium
Consider Bartter or Gitelman syndrome, especially with longstanding disease or family history

Helpful patterns:

Gitelman syndrome: hypomagnesemia, hypocalciuria, thiazide-like phenotype.
Bartter syndrome: often hypercalciuria, loop-diuretic-like phenotype.
Loop/thiazide diuretics: temporal medication relationship and positive urine screen.

6. Evaluate potassium and magnesium in every case

Hypokalemia both accompanies and perpetuates metabolic alkalosis. Check:

Serum K and Mg
Urine K if distinguishing renal from extrarenal K loss is needed
ECG if K is significantly low or symptoms are present

Correcting potassium chloride, and magnesium when low, is part of diagnostic stabilization as well as treatment.

Compact algorithm

High serum HCO3- ± alkalemia
        ↓
ABG/VBG: confirm primary metabolic alkalosis and expected compensation
        ↓
History, volume status, BP, K/Mg/Cr
        ↓
Spot urine chloride
   ┌───────────────────────────────────────┐
   │ <20 mmol/L                            │ ≥20 mmol/L
   │ chloride-responsive                   │ chloride-resistant/renal loss
   │ vomiting, NG loss, remote diuretics   │
   └───────────────┬───────────────────────┘
                   │
      With urine Cl ≥20 mmol/L:
        ├─ Hypertension → renin + aldosterone, assess mineralocorticoid states
        └─ Normal/low BP → active diuretic screen, Mg, urine Ca, Bartter/Gitelman

Urgent features

Escalate promptly if there is severe alkalemia, significant arrhythmia risk, seizures/tetany, altered mental status, severe hypokalemia, or renal failure. Severe alkalemia can lower ionized calcium and worsen cardiac and neuromuscular complications.

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