Falsely low pO₂(a) – a case study of spurious hypoxemia

When blood gas analysis reveals a reduced pO₂(a) in a patient without any immediate clinical evidence of hypoxemia, consideration should be given to the possibility that the result is falsely low, allowing a diagnosis of spurious or pseudo-hypoxemia. 

Technical deficiencies (e.g. sampling of venous rather than arterial blood, undue delay between sampling and measurement, analyzer malfunction, etc.), may account for spurious hypoxemia in particular cases. However, as a recently published case study highlights, spurious hypoxemia may be caused by pre-existing disease. 

A 49-year-old man diagnosed with chronic myeloid leukemia seven years previously was admitted to hospital for surgical treatment of serious acute complications. Postoperative infection and resulting sepsis led to a decision to temporarily withdraw the chemotherapy that was being used to treat leukemia. 

As a consequence, white-cell count rapidly rose from 20,0000/mm³ to a peak of 144,000/mm³ over the following 6 days. During the same time period, measured pO₂(a) fell from 25 kPa to 5.6 kPa (188 mmHg to 42 mmHg), indicating severe hypoxemia. 

The patient, however, was not cyanotic and despite intensive investigation, no clinical explanation for hypoxemia was identified. Furthermore, throughout this 6-day period, as pO₂(a) fell precipitantly, oxygen saturation, measured by pulse oximetry, remained normal (97 %). Evidence suggested that despite a marked reduction in pO₂(a), oxygen transport and delivery was normal. 

A presumptive diagnosis of spurious hypoxemia was confirmed when high-dose chemotherapy was reinstated. As white-cell count quickly fell towards normal over the following week in response to chemotherapy, pO₂(a) also returned to normal. The massive increase in white-cell numbers (hyperleukocytosis) was the cause of spurious hypoxemia in this case. 

White blood cells continue to consume oxygen after blood has been collected. The higher the white-cell count, the greater is the rate of this in vitro oxygen consumption.