Metabolic acidosis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

Metabolic acidosis is present when the blood bicarbonate concentration is decreased (<24 meq/L). In order to determine the underlying etiology of the metabolic acidosis, the following laboratory measurements are needed to specify whether metabolic acidosis is an isolated or combined process and to calculate the plasma anion gap:

• Plasma bicarbonate
• PCO2
• Arterial pH
• Plasma Na+
• Plasma Cl-

In normal anion gap metabolic acidosis, additional measurement of urinary Na+, K+, and Cl- is needed in order to calculate urinary anion gap.

In high anion gap metabolic acidosis, the osmolal gap needs to be calculated; and therefore, blood sodium concentration, blood glucose, and BUN are needed.

Shown below is an algorithm depicting the series of laboratory tests needed to evaluate metabolic acidosis.

❑ Anion Gap (Na+ - Cl- - HCO3-) ❑ Consider measurement of albumin, Ca2+, K+, and Mg2+)

High anion gap ❑ Screen for ketonuria (dipstick acetoactetae or plasma beta hydoxybutarate) ❑ Renal function ❑ Lactate concentration ❑ Toxin screen ❑ Osmolal gap

Normal-low anion gap ❑ Urinary anion gap (Na+ + K+ - Cl-)

Pure vs Combined Metabolic Acidosis

Pure vs combined metabolic acidosis can be estimated using the following values: plasma HCO3^-, PCO2, and arterial pH. Shown below is a table summarizing the findings in the different scenarios.

Acid base status	Plasma bicarbonate (meq/L)	Arterial pH	PCO2 (mmHg)
Normal	24	7.4	40
Pure metabolic acidosis	↓	↓	↓
Combined metabolic and respiratory acidosis	↓	↓	↔
Combined metabolic and respiratory alkalosis	↓	↔	↓

Pure high anion gap metabolic acidosis can be differentiated from combined metabolic acidosis by using the following equation: Δ Anion gap (AG)/ Δ HCO3^-

Δ AG/ Δ HCO3-	Interpretation
1-2	Pure high anion gap metabolic acidosis
<1	High anion gap metabolic acidosis PLUS metabolic acidosis
>2	High anion gap metabolic acidosis PLUS metabolic alkalosis

The following laboratory studies should be considered:

• Anion gap
• Arterial blood gas sampling
• ECG (to assess for arrhythmias)
• Electrolytes basic metabolic panel), complete blood count.
• Imaging of the kidneys
• Serum lactate, ketone
• Toxicological screening, salicylate level (methanol or ethylene glycol)
• Urinalysis can reveal acidity, (salicylate poisoning) or alkalinity (renal tubular acidosis type I), and ketones in ketoacidosis.