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Wednesday, 30 June 2021

THE IMPORTANCE OF ANION GAP

Written by Esperanza Cuadrado Galván | Luz del Mar Rivas Chacón | Gema Sánchez Helguera

Figure 1. A 48-year-old- woman was admitted to the major burned unit for extensive burn injury about 72 % of total body surface área. During the hospital stay we received in the laboratory a green-colored urine. In the follow-up of the patient, extreme values of anion clohride (Dimension Vista 1500, SIEMENS) and anion GAP were obtained, therefore the laboratory clinician studied the clinical history of the patient as the prescribed medication to find the possible cause of these results.

A 48-year-old- woman was admitted to the major burned unit for extensive burn injury about 72 % of total body surface area. In the follow-up of the patient during her stay in the hospital the following results of anion clohride (Dimension Vista 1500, SIEMENS) and anion GAP were obtained (Table 1).
The most common equations for anion gap estimations are:

Anion Gap (AG) = Sodium - (Chloride + Bicarbonate)
Anion Gap (AG) = (Sodium + Potassium) – (Chloride + Bicarbonate)

The clinicians in charge of the patient contacted to the laboratory specialist because they did not find a cause for this anion gap values, doubting about the reliability of the results. The patient was being treated with cerium nitrate (possible cause of the methemoglobinemia). Examination of the co-oximetry parameters, revealed the following results of methemoglobin measured by ABL 90 FLEX RADIOMETER analyzer (Table 2).

imagen2Table 1. Results of anion clohride and anion GAP

To treat the methemoglobinemia the methylene blue was prescribed to the patient. By the time, we received a green colored urine for biochemistry determinations. Asociating the green color of the urine to the methylene blue treatment and evaluating the results of anion gap, the lab informed that the hyperchloremia could by caused by the methylene blue which chemical formula is methylthioninium chloride.

imagen3Table 2. Results of methemoglobin

The anion gap is a calculated value used to identify the cause of metabolic acidosis. This parameter provides information about the unmeasured anions but do not give information about the cause of the imbalance. An increased anion gap could indicate an exogenous administration of some compound that interfere in anion results in serum samples. The interpretation of chloride levels in patients with methylene blue treatment, the interference of this compound should be considered. In these patients we can find hyperchloremia with an elevated anion gap. The laboratory specialist should be always in contact to clinicians for an accurate interpretation of the results.

REFERENCE

  1. Do Nascimento TS, Pereira RO, de Mello HL, Costa J. Methemoglobinemia: from diagnosis to treatment. Rev Bras Anestesiol. 2008;58(6):651‐664.
  2. Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57‐64.
  3. Nogué Xarau S, Aguilar Salmerón R, Corominas García N, Soy Muner D. Metahemoglobinemia, azul de metileno y stock de antídotos [Methahemoglobinemia, methylene blue and antidote stockpile]. Med Clin (Barc). 2009;132(13):526

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