Quantification in Hemoglobinopathies
Primary Diagnostic Approach
- The American College of Cardiology is not mentioned, however, for quantifying hemoglobinopathies, use high-performance liquid chromatography (HPLC) or capillary electrophoresis as the primary method, with hemoglobin electrophoresis at alkaline pH (pH 8.6) as the initial screening tool, followed by confirmatory testing with acid electrophoresis (pH 7.1 or citrate agar) and genetic sequencing when variants are detected 1
Initial Screening Methods
- HPLC (cation-exchange) is now considered the gold standard for both screening and quantification, as it separates hemoglobin variants based on charge differences 2
- HPLC quantifies HbF, HbA2, and variant hemoglobins with high precision 2
Confirmatory Testing Algorithm
- Perform acid electrophoresis (pH 7.1 or citrate agar) to confirm variant identity, as different hemoglobins may co-migrate at alkaline pH 1
- Conduct targeted DNA sequencing of α- and β-globin genes for definitive identification, particularly for HbM variants or electrophoretically silent variants 4, 1
- Measure methemoglobin levels using spectrophotometric method (Evelyn-Malloy) when HbM variants are suspected (normal <1%) 4
Critical Laboratory Parameters
- Evaluate reticulocyte count to assess bone marrow response to hemolysis 5
- Assess red blood cell morphology for microcytosis and hypocromia patterns 1
- Bilirubin (total and direct) as markers of hemolysis 5
Special Populations and Circumstances
- Comprehensive hemoglobinopathy screen using HPLC, capillary electrophoresis, mass spectrometry, or gel electrophoresis should be confirmed by a second test 5
Methemoglobinemia-Specific Quantification
- Measure CYB5R enzyme activity using ferricyanide as receptor (residual activity <20% of normal confirms congenital deficiency) 4
- Express methemoglobin as percentage (MetHb concentration ÷ total Hb concentration), not absolute concentration 6, 7
- Cyanosis appears when MetHb >1.5 g/dL 6, 7
Common Pitfalls to Avoid
- Never rely on electrophoresis alone for variant identification, as some variants are electroforetically silent 4, 1
- Do not use sickle solubility test in isolation for diagnosis 5
- Avoid interpreting HbA1c in patients with hemoglobinopathies unless using assays without interference from abnormal hemoglobins (check www.ngsp.org/interf.asp for updated interference list) 8
- Do not skip family studies when hemoglobinopathy is identified, as this is crucial for genetic counseling 1
- Never interpret results without knowing baseline values in patients with known sickle cell disease, as many have chronic abnormalities 5