Calculadora do Gap Aniônico - Avaliação do Equilíbrio Ácido-Base

The anion gap is a bookkeeping check on the electrolytes in blood. Measured cations and measured anions never balance exactly, because routine panels do not measure every charged particle present; the residual difference is the anion gap. A gap that has widened is one of the fastest ways to narrow down the cause of a metabolic acidosis at the bedside, because it separates acidosis caused by an accumulating acid from acidosis caused by losing bicarbonate.

A fórmula

Anion gap = Na+ - (Cl- + HCO3-)
Corrected AG = AG + 2.5 x (4.0 - albumin in g/dL)
Delta ratio = (AG - 12) / (24 - HCO3-)

• Na+ = serum sodium in mmol/L
• Cl- = serum chloride in mmol/L
• HCO3- = serum bicarbonate in mmol/L
• Albumin correction is applied because albumin is the largest unmeasured anion

The three-electrolyte form is the version in general clinical use; some centres add potassium, which raises the expected range by about 4 mmol/L. The albumin correction of 2.5 mmol/L per 1 g/dL below normal follows Figge et al., Journal of Laboratory and Clinical Medicine, 1998.

How the calculation works

Blood is electrically neutral, so the total positive charge equals the total negative charge. A routine panel measures one major cation, sodium, and two major anions, chloride and bicarbonate. Subtracting the measured anions from the measured cation leaves a positive residue that represents the unmeasured anions, chiefly albumin, phosphate, sulfate and organic acids, minus the unmeasured cations such as potassium, calcium and magnesium.

When an acid accumulates in the blood, its hydrogen ion is buffered by bicarbonate, which falls, while the acid anion that came with it is not measured by the panel. Chloride does not change to compensate. The result is a widened gap. If instead the body loses bicarbonate directly, through diarrhoea or renal tubular acidosis, the kidneys retain chloride to preserve neutrality, bicarbonate and chloride move in opposite directions by the same amount, and the gap stays normal.

How to read your result

Reference ranges depend on the laboratory. Older ion-selective electrode methods gave a normal range around 8 to 16 mmol/L; contemporary analysers measure chloride slightly higher and typically produce a normal range of about 3 to 11 mmol/L. Because of this drift, the gap should always be compared against the reporting laboratory range rather than a remembered figure.

• The classic causes of a high gap are captured by the mnemonic GOLD MARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.
• Every 1 g/dL fall in albumin below 4 g/dL lowers the measured gap by roughly 2.5 mmol/L, so an unwell patient with low albumin can have a genuinely raised gap that reads as normal.

The delta ratio and mixed disorders

In a pure high anion gap acidosis, the gap rises by about as much as bicarbonate falls, because each accumulated acid anion displaces one bicarbonate. Comparing the two changes as a ratio therefore tests whether a second acid-base disorder is hiding underneath. A ratio near 1 to 2 is consistent with a single high gap acidosis.

A ratio below 1 suggests bicarbonate has fallen further than the gap has risen, pointing to a coexisting normal gap acidosis. A ratio above 2 suggests bicarbonate is higher than the gap alone would predict, pointing to a coexisting metabolic alkalosis or a compensated chronic respiratory acidosis. The delta ratio is a screening heuristic and performs poorly at the extremes, so it is used to raise questions rather than settle them.

Limitações

• Reference ranges differ substantially between laboratories because chloride methods differ. A gap must be read against the range printed on the report that produced it.
• Hypoalbuminaemia masks a raised gap. Without the albumin correction, a critically ill patient with an albumin of 2 g/dL can show a normal-looking gap while a significant acid load is present.
• The gap identifies a pattern, not a diagnosis. A raised gap narrows the differential but says nothing about which acid is accumulating; that requires lactate, ketones, osmolar gap, salicylate level or renal function.
• Severe hyperlipidaemia or paraproteinaemia can interfere with sodium measurement by some methods, distorting the gap in either direction.
• The delta ratio assumes a one-to-one exchange between the accumulating anion and bicarbonate, which holds only approximately, and it becomes unreliable when the gap is only mildly raised.
• This calculator arithmetically combines values you enter. It cannot detect a transcription error, and it is not a substitute for clinical interpretation of a blood gas alongside the patient.

Perguntas frequentes

What is a normal anion gap?

Roughly 3 to 11 mmol/L on contemporary analysers, or about 8 to 16 mmol/L on older methods, when calculated without potassium. Adding potassium raises the expected range by about 4 mmol/L. Always compare against the reporting laboratory range.

Why correct the anion gap for albumin?

Albumin is negatively charged and is the largest single contributor to the unmeasured anions. When albumin is low, the gap falls for reasons unrelated to acid, which can hide a real high gap acidosis. The correction adds back about 2.5 mmol/L for every 1 g/dL that albumin sits below 4 g/dL.

What does a high anion gap mean?

It means an unmeasured acid anion is accumulating, most often lactate, ketoacids, or products of kidney failure or a toxic alcohol ingestion. It narrows the differential but does not identify which acid; that needs specific tests.

Can the anion gap be negative?

Yes, though it is uncommon. A negative gap usually reflects a laboratory interference, such as bromide or iodide poisoning, severe hyperlipidaemia, or a paraprotein, or a marked increase in unmeasured cations such as in severe hypercalcaemia or lithium toxicity.

Does a normal anion gap rule out acidosis?

No. Acidosis from bicarbonate loss, such as severe diarrhoea or renal tubular acidosis, produces a normal gap because chloride rises as bicarbonate falls. A normal gap distinguishes the type of acidosis; it does not exclude one.

Referências

1. Hopkins E, Sanvictores T, Sharma S. Physiology, Acid Base Balance. StatPearls, NCBI Bookshelf.
2. Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Critical Care Medicine, 1998.
3. Mehta AN, Emmett JB, Emmett M. GOLD MARK: an anion gap mnemonic for the 21st century. The Lancet, 2008.
4. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clinical Journal of the American Society of Nephrology, 2007.