Our first case illustrated how false-positive lactate might have led to misdiagnosis, inappropriate laparotomy and delayed treatment. The treatment of ethylene glycol ingestion is time-dependant; delays could lead to serious consequences, including renal failure and death. Equally, laparotomy, in a patient with severe acidosis, could be associated with considerable perioperative risk. In contrast, our second case demonstrated how discrepant lactate readings can instead be used as a diagnostic clue to actually expedite diagnosis and treatment.
Our experiment revealed false-positive lactate elevation readings for 2 major ethylene glycol metabolites, glycolate and glyoxalate, but only with the Radiometer Analyzer. We have shown how results from the Radiometer that contrast with those from either the i-STAT or Bayer point-of-care analyzers can be used rapidly at the bedside, hours ahead of confirmatory laboratory results, to indicate ethylene glycol poisoning.
Most L-lactate analyzers use L-lactate oxidase. The false-positive results from the Radiometer likely occur because ethylene glycol metabolites cross-react with L-lactate oxidase.2–4
In contrast, ethylene glycol metabolites cause minimal lactate elevation with the Bayer, iSTAT and Vitros devices, to a degree unlikely to cause misdiagnosis. Of note is that, in addition to antifreeze, ethylene glycol is found in many common products.6
Our experiment showed that even modest ingestion (causing glycolate and glyoxylate concentrations in blood of 5–10 mmol/L) could produce false-positive lactate results of 10–20 mmol/L with the Radiometer Analyzer ().
Patients who come to hospital late after ethylene glycol ingestion are among the sickest from high metabolite concentrations. However, they are difficult to confirm diagnostically because plasma ethylene glycol concentration and osmolar gap may become normal upon metabolism. Most hospitals measure ethylene glycol, not its metabolites. The lactate gap offers a surrogate test for metabolites (glyoxalate and glycolate) that is useful in diagnosis late presentations. Futhermore, most dialysis protocols rely upon serial measurements of plasma concentrations of ethylene glycol, even though its toxicity results from its metabolites. As long as a lactate gap exists, metabolites still exist. Treatment with dialysis could be ended with confidence, once the gap has disappeared.
Lactate is key in the diagnosis of mesenteric ischemia, and in decisions about laparotomy, because of a sensitivity approaching 100% (and despite specificities of 42%– 87%).7,8
The benefits of point-of-care analyzers include rapid turn-around and low cost. Point-of-care lactate testing is therefore increasingly common, and the Radiometer Analyzer is popular. The potential for misdiagnosis is therefore substantial, especially since patients may deny ethylene-glycol ingestion; such denials might not be doubted, given an elevated lactate result.9,10
Clinicians know that quantitative laboratory errors occur, and will therefore repeat suspicious values. However, artifactual errors that require retesting with a different machine are less commonly appreciated.
The lactate gap1
is a quick and sensitive test that can be established at the point of care. We showed how a false-positive point-of-care lactate could mean serious misdiagnosis. However, we also showed this to be a diagnostic aid to expedite faster treatment. In short, clinicians should “Mind the Gap.”