The role of the clinical laboratory in diagnosing acid–base disorders

Abstract

Acid–base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body’s acid–base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid–base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid–base disorder is generally responsible for a patient’s signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid–base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid–base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid–base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid–base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid–base disorders.

Keywords:

• pH
• blood gas
• pCO₂
• bicarbonate
• base excess
• alveolar–arterial-gradient
• lactate
• lactate gap
• anion gap
• toxic alcohols
• osmolal gap
• urine anion gap
• urine electrolytes

Acknowledgements

We are indebted to Jan-Willem Boldingh for making the gamblegram (Figure 3).

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 Note: the terms serum and plasma are used largely interchangeably in this paper.

2 Measured osmolality has units of mmol/kg while calculated osmolarity has units of mmol/L. Thus, the osmolal gap calculation (the difference between measured osmolality and calculated osmolarity) has mixed units. The difference in the values of these two parameters that can be attributed to the difference in units is negligible in a clinical setting. Thus, when the osmolal gap is calculated, the units are ignored.