Clinical Information

Hemoglobin is contained within erythrocytes and significant amounts of "free hemoglobin" (outside the red blood cell [RBC]) are not normally present in plasma. This free hemoglobin is also called plasma hemoglobin.

Normal blood draw procedures cause a limited degree of unavoidable disruption and therefore a small amount of free hemoglobin may normally be present. When detectable, the total plasma hemoglobin and a subcomponent, oxyhemoglobin, are both reported.

Significant amounts of free hemoglobin occur in plasma following disruption of the RBC for any reason. This might result from a transfusion reaction or mechanical fragmentation of RBCs due to instrumentation, surgical procedures, or mechanical devices. Patients requiring support from extracorporeal membrane oxygenation or centrifugal ventricular assist devices are commonly monitored for trends in plasma free hemoglobin levels to assess for increasing hemolysis. Sharp spikes in plasma hemoglobin levels can indicate pump disruption. However, plasma hemoglobin can be artifactually increased due to a traumatic blood draw or prolonged exposure to post-draw RBCs. Additionally, bilirubin interferes substantially with the ability to calculate total plasma hemoglobin levels and results may be spurious and unreliable. This is a difficulty frequently encountered in serially tested patients. When this occurs, the oxyhemoglobin level tends to show less interference and will be the only analyte reported in the presence of increased bilirubin (>5 mg/dL). When using trending data, total plasma hemoglobin and oxyhemoglobin levels are not interchangeable and should be compared within their subgroups only.