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5.7: Assessment

  • Page ID
    11043
  • Main Aspects of Assessment

    The three aspects of assessment of this acid-base disorder are:

    • First: Recognise its presence
    • Second: Diagnose the cause
    • Finally: Measure the severity

    5.7.1 Investigations

    A metabolic acidosis is often strongly suspected because of the clinical presentation of the patient (eg diabetes, renal failure, severe diarrhoea). Three clues from a typical hospital automated biochemical profile are:

    • Low bicarbonate (or low total CO2)
    • High chloride
    • High anion gap

    What is total CO2?

    This is often reported as part of the laboratory's automated biochemical profile on a venous blood sample. It represents the total concentration of all the species in the sample which can be converted to carbon dioxide gas. This is:

    \( \text {Total } CO_{2} = [HCO_{3}^{-}] + [H_{2}CO_{3}] + [\text {carbamino } CO_{2}] + [\text {dissolved } CO_{2}] \)

    Apart from bicarbonate, all the other species are present in only small concentrations. The usefulness of the 'total CO2' is as an estimate of the arterial bicarbonate & which can be obtained without collecting an arterial sample. The value will usually be several mmols/liter higher than the actual arterial value due to the inclusion of carbamino & dissolved CO2 and because of the higher CO2 content of venous blood.

    Arterial blood gases are important for diagnosis but should always be interpreted in conjunction with the clinical details.

    In addition to arterial blood gases, some other investigations useful for indicating a metabolic acidosis and for differentiating between the various major causes are:

    • Urine tests for glucose and ketones
    • Electrolytes (incl chloride, anion gap, bicarbonate)
    • Plasma glucose
    • Urea and creatinine
    • Lactate

    5.7.2: Use of Ancillary Indices

    There are several indices (which can be calculated from pathology results) which may be useful in assessing a metabolic acidosis:

    • Anion gap
    • Delta ratio
    • Urinary anion gap
    • Osmolar gap

    The anion gap is useful in a couple of ways:

    • Alerting Role: An elevated anion gap (esp if AG > 20 mmol/l) will alert the clinician to the presence of a high anion gap metabolic acidosis. This can be extremely useful in sorting out complicated mixed disorders.
    • Classification Role: It is used to divide metabolic acidosis into two major subgroups. The next step then is to consider either the 4 major categories of high anion gap acidosis (ketoacidosis, lactic acidosis, uraemic acidosis, acidosis due toxins) or the 2 major categories of normal anion gap acidosis (renal group, GIT group). History and a few pertinent investigations will usually distinguish the cause.

    The delta ratio can be useful particularly in the difficult situation of a metabolic acidosis due to two processes where one elevates the anion gap and the other does not. An example is the hyperchloraemic normal anion gap acidosis which may develop in patients who have diabetic ketoacidosis (high anion gap). The ratio gives an indication of the relative contribution of the two processes. Unfortunately, its interpretation is limited somewhat by the wide error margin in this derived variable.

    The urinary anion gap and the osmolar gap may be useful in certain patients with acidosis.