False Positive Rate of Carbon Monoxide Saturation by Pulse Oximetry of Emergency Department Patients

Abstract

BACKGROUND:

Symptoms of carbon monoxide (CO) poisoning are non-specific. Diagnosis requires suspicion of exposure, confirmed by measuring ambient CO levels or carboxyhemoglobin (COHb). An FDA-approved pulse oximeter (Rad-57) can measure CO saturation (S_pCO). The device accuracy has implications for clinical decision-making.

METHODS:

From April 1 to August 15, 2008, study personnel measured S_pCO and documented demographic factors at time of clinical blood draw, in a convenience sample of 1,363 subjects presenting to the emergency department at Intermountain Medical Center, Murray, Utah. The technician then assayed COHb. COHb and S_pCO values were compared by subject; false positive or negative values were defined as S_pCO at least 3 percentage points greater or less than COHb level, reported by the manufacturer to be ± 1 SD in performance.

RESULTS:

In 1,363 subjects, 613 (45%) were male, 1,141 (84%) were light-skinned, 14 in shock, 4 with CO poisoning, and 122 (9%) met the criteria for a false positive value (range 3–19 percentage points), while 247 (18%) met the criteria for a false negative value (−13 to −3 percentage points). Risks for a false positive S_pCO reading included being female and having a lower perfusion index. Methemoglobin, body temperature, and blood pressure also appear to influence the S_pCO accuracy. There was variability among monitors, possibly related to technician technique, as rotation of monitors among technicians was not enforced.

CONCLUSIONS:

While the Rad-57 pulse oximeter functioned within the manufacturer's specifications, clinicians using the Rad-57 should expect some S_pCO readings to be significantly higher or lower than COHb measurements, and should not use S_pCO to direct triage or patient management. An elevated S_pCO could broaden the diagnosis of CO poisoning in patients with non-specific symptoms. However, a negative S_pCO level in patients suspected of having CO poisoning should never rule out CO poisoning, and should always be confirmed by COHb.

Keywords

carbon monoxide carbon monoxide poisoning pulse oximetry carboxyhemoglobin false positive

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