Hemoglobin C Disease: Overview, Clinical Presentation, Laboratory ...

Patients can usually be evaluated in an outpatient setting. Routine screening laboratory studies are not necessary for asymptomatic patients; those with a moderate to severe clinical course should be evaluated for other hemoglobinopathies or chronic conditions.

The most commonly used method for the detection of hemoglobinopathies is hemoglobin electrophoresis or high-performance liquid chromatography (HPLC). Cellulose acetate screening will distinguish between the normal hemoglobins HbA, HbF, and HbA2, as well as the common variants, Hb S and Hb C, by charge. In the United States, newborn screening is the most common method of detecting individuals with Hb C disease. For those not diagnosed at birth, hemoglobin variant analysis by HPLC can provide the diagnosis.

Hemoglobin analysis results are as follows:

• Patients who are homozygous for Hb C show mostly Hb C; Hb A is absent and Hb F is slightly increased compared with normal individuals.
• Patients who are heterozygous for Hb C may show 30-40% Hb C, 50-60% Hb A; Hb A2 is increased slightly.
• Patients who have hemoglobin C and beta-zero thalassemia show no hemoglobin A; to distinguish these patients from homozygous C patients, the best method is to test both parents, if possible.
• Patients who have Hb C and beta+ thalassemia show low but present levels of hemoglobin A.

Hemoglobin variants that have the same mobility as Hb C include Hb E and Hb O-Arab. If co-migration is suspected, usually a second electrophoretic procedure is performed on citrate agar (acid electrophoresis) which is capable of distinguishing the respective variants.

In pregnancy, if prenatal screening suggests an increased risk of hemoglobinopathy, hemoglobin analysis is indicated for the mother. If maternal hemoglobin analysis shows that the mother is either homozygous or a heterozygous carrier for Hb C, paternal evaluation may be indicated to assess the fetal risk of Hb SC disease. DNA-based testing for hemoglobin C can be performed during the first trimester of pregnancy from chorionic villus sampling at 10-12 weeks gestation or by amniocentesis after 15 weeks gestation.

HemeChip, a miniaturized paper-based microchip electrophoresis hemoglobin test, has a reported sensitivity of 100% and an accuracy of 98.4% for identifying hemoglobin variants. It has been proposed as a point-of-care option for screening newborns in low-resource settings. [25] However, the initial cost to acquire the machine and the continuous power supply needed to run the test may limit its usability. [26]

An alternative test that does not require instrumentation, electricity, or refrigeration, HemoTypeSC™ has also been studied for point-of-care screening in low-resource communities. This test is a competitive lateral-flow immunoassay that uses monoclonal antibodies to detect hemoglobins A, S, and C in a 1.5-μL sample of whole blood. A global, multicenter evaluation reported an overall sensitivity of 99.5% and specificity of 99.9% across all hemoglobin phenotypes. [26]

Hemoglobin C Trait

Hemoglobin concentrations are usually within the normal range, but the erythrocyte mass and survival may both be decreased. [1] Despite decreased survival, the reticulocyte count will not typically be increased; this is thought to be secondary to the lower oxygen affinity of Hb C, which facilitates sufficient oxygen delivery to tissues despite a lower erythrocyte mass. Peripheral blood may show moderate amounts of target cells and intracellular crystals (5-30%). [24, 27]

Hemoglobin C Disease

Hemolytic anemia may be mild to moderate in severity. Markers of hemolysis include increased LDH, reticulocyte count, and direct bilirubin [1] . Despite the anemia, reticulocyte counts are only slightly elevated in homozygous individuals, due to the reduced oxygen affinity of this hemoglobin variant. Erythrocyte morphology is markedly abnormal, with prevalent microcytosis, target cells (>90%), spherocytes, and crystallized hemoglobin. [28, 27, 29]

The reticulocyte count in these patients may be normal despite the presence of chronic hemolytic anemia; this is proposed to be due to a reduced affinity of Hb C for oxygen. The lower affinity facilitates sufficient delivery of oxygen to tissues in the face of mild anemia.

The electrostatic interactions between the positively charged β-6 amino groups and the negatively charged groups on adjacent molecules lead to decreased solubility of deoxy-Hb C. This decreased solubility likely leads to shortened erythrocyte survival. In settings of hypertonicity or deoxygenation, Hb CC cells form intracellular crystals of hemoglobin that are classically hexagonal in shape. These crystals reduce the internal viscosity of the erythrocyte, leading to reduced deformability and a predisposition to fragmentation, spherocyte formation, and splenic sequestration. [29]