Sickle cell disease refers to individuals who are homozygous for the sickle cell mutation (Hb SS), or compound heterozygous for the sickle cell mutation and other hemoglobin variants such as Hb C, Hb D-LA, Hb O-Arab, or beta thalassemia mutations (Hb S beta thalassemia). The pathophysiology of sickle cell disease (SCD) is characterized by intermittent vaso-occlusive events and chronic hemolytic anemia. One of the earliest manifestations of SCD in young children is dactylitis, or “handfoot syndrome.” Common problems in children include intermittent vaso-occlusive pain episodes, febrile events, acute chest syndrome, splenic sequestration and aplastic episodes due to infection with parvovirus. Stroke is one of the most devastating complications of SCD as a result of stenosis of the large intracranial arteries. Ongoing hemolysis and vaso-occlusion affects virtually every organ system and may manifest in older patients as a chronic pain syndrome, cholelithiasis, osteonecrosis, retinopathy, renal disease, priapism, chronic lung disease and pulmonary hypertension.
Accurate diagnosis based on identification of different structural hemoglobin variants or thalassemia mutations is therefore essential for optimal management of clinically significant hemoglobin disorders. In California, newborns are screened for sickle cell disease, HbH disease and beta-thalassemia. The CHRCO Hemoglobinopathy Laboratory serves as the central reference laboratory for the California State Newborn Screening Program to confirm all positive primary screening results.
Most of the common Hb variants, such as Hb S, Hb C, and Hb E can be identified using a combination of chromatographic and electrophoretic methods.
Automated cation exchange HPLC (CE-HPLC) is a rapid and sensitive technique for the presumptive identification of many abnormal hemoglobin variants and for quantitation of HbA2 and Hb F. It is the method of choice used by the California state newborn screening program to identify hemoglobin disorders. Electrophoretic methods, including IEF and more recently, capillary IEF (cIEF), are employed by the CHRCO Hemoglobinopathy Reference Laboratory as part of a panel of complementary techniques to resolve or confirm Hb variants.
Definitive diagnosis of a hemoglobin disorder, particularly thalassemia, may require DNA analysis. The strategy used to identify uncommon Hb variants or thalassemia mutations is to first test a panel of mutations common to the population being screened, with further testing for rare mutations based on clinical suspicion, family history and ethnic background.
Alpha globin genotyping using gap-PCR is used to diagnose the most common deletional forms of alpha thalassemia and distinguish HbH disease from alpha thalassemia trait. Direct DNA sequencing and other molecular methods such as MLPAare required to diagnose hemoglobinopathies that are suspected by laboratory or clinical findings.In addition to the basic tests used for detection of hemoglobin disorders, the CHRCO Hemoglobinopathy Reference Laboratory performs more specialized tests, including tests for unstable hemoglobins(heat denaturation), methemoglobin (spectrometry), oxygen carrying capacity (oxygen dissociation curve), and red cell membrane abnormalities.