Sickle Cell Disease (SCD)


Sickle cell disease (SCD) is an autosomal recessive condition encompassing several abnormalities of the beta-globin gene. Patients are either homozygous for hemoglobin S (HbS; βSS) or have compound heterozygosity with a βS allele plus another mutant beta-globin gene, such as thalassemia (eg, βS0). The abnormal HbS is less hydrophilic than normal adult hemoglobin (HbA) and polymerizes easily. Polymerization of HbS molecules deforms the shape of erythrocytes. 

Testing for SCD depends on the age of the patient. 

Prenatal Testing 

Testing in the prenatal setting involves DNA-based methods because the predominant form of hemoglobin in utero, fetal hemoglobin (HbF), does not contain a beta-globin subunit; rather, it has 2 alpha subunits and 2 gamma subunits (α2γ2).1 Thus, DNA-based methods are more appropriate than solubility testing or hemoglobin electrophoresis. Prenatal testing involves chorionic villus sampling between weeks 8 and 10 of gestation.2 Techniques that have been developed include restriction analysis,3 allele-specific hybridization,4 reverse dot blotting,5 and fluorescent polymerase chain reaction (PCR).6 

Newborn Screening

New York was the first state to implement newborn screening (NBS) for SCD.7 In 2008, the United States Preventive Services Task Force (USPSTF) officially recommended that all newborns be screened for SCD with thin-layer isoelectric focusing or high-performance liquid chromatography (HPLC) because of the high-level sensitivity and specificity of these tests for detecting SCD.8 NBS is currently mandatory in all 50 states and the District of Columbia. Newborns should be screened regardless of birth setting, and physicians should verify that an infant has undergone NBS at the first office visit. Blood samples are obtained either by heel stick or directly from the umbilical cord; these are then spotted on a filter paper for transportation. 

If the result is positive, confirmation should be arranged at no later than 2 months of age.8 Early diagnosis and early intervention are some of the most important aspects of the care of patients with SCD and have reduced childhood mortality due to the disease.7 Early intervention involves penicillin prophylaxis and vaccination to prevent infections—a major cause of mortality in patients younger than 5 years old.8,9 

Test results are interpreted according to hemoglobin patterns named after the hemoglobin types encountered in the sample. For example, an FA pattern describes a sample containing HbF and HbA. Patterns are named with the predominant hemoglobin type listed first, and the rest in descending order. They are summarized below10:

  • FS pattern: SCD is associated with this pattern. The predominant hemoglobin subtype in newborns is HbF (lacks the beta-globin subunit), with a small amount of HbS. If this pattern is reported, SCD should be confirmed with a more specific test, such as hemoglobin electrophoresis. Infants with sickle cell β-thalassemia also are not able to produce the beta-globin subunit and therefore display an FS pattern, which further underscores the importance of confirmatory testing. Other possibilities compatible with an FS pattern are sickle beta plus thalassemia (Sβ⁺) and sickle cell with hereditary persistence of fetal hemoglobin (HPFH). 
  • FSA pattern: This specimen contains HbF  >  HbS and a small amount of HbA. Possibilities include sickle cell trait (SCT) and sickle beta plus thalassemia. 
  • FSC pattern: This specimen contains HbF > HbS > HbC. The patient most likely has hemoglobin SC disease—a milder form of SCD. 
  • Other patterns: NBS may also report FSD and FSE patterns, consistent with HbSD and HbSE disease, respectively. An FSV pattern refers to a sample with HbF, HbS, and an unidentified variant. 

Patients with a positive test result should immediately begin receiving penicillin prophylaxis. They should be educated and referred to a specialist in pediatric hematology/oncology. Patients with SCT may be reassured, and genetic testing of family members may be offered. 

Hemoglobin proteins are classically quantified with electrophoretic methods, which include capillary electrophoresis, cellulose acetate electrophoresis, agar gel electrophoresis, and isoelectric focusing. This last method separates hemoglobin variants on the basis of charge at a given pH and is a useful confirmatory test.11

Older Children and Adolescents

Disease may go undetected in children who were born in countries that do not implement adequate NBS and then emigrate, so that they do not benefit from NBS.12 In this patient population, high-performance liquid chromatography (HPLC) is preferred for its excellent sensitivity, specificity, and ability to provide qualitative and quantitative information.13 Isoelectric focusing may also be used and is currently more cost-effective.14 

References

1. Kaufman DP, Khattar J, Lappin SL. Physiology, fetal hemoglobin. StatPearls [Internet]. Updated March 29, 2021. Accessed November 16, 2021.

2. Malcolm E. Sickle cell disease and pregnancy. Sickle Cell Disease News. Accessed November 11, 2021. 

3. Embury SH, Scharf SJ, Saiki RK, et al. Rapid prenatal diagnosis of sickle cell anemia by a new method of DNA analysis. N Engl J Med. 1987;316(11):656-661. doi:10.1056/NEJM198703123161103

4. Saiki RK, Bugawan TL, Horn GT, Mullis KB, Erlich HA. Analysis of enzymatically amplified beta-globin and HLA-DQ alpha DNA with allele-specific oligonucleotide probes. Nature. 1986;324(6093):163-166. doi:10.1038/324163a0

5. EL-Fadaly N, Abd-Elhameed A, Abd-Elbar E, El-Shanshory M. Accuracy of reverse dot-blot PCR in detection of different β-globin gene mutations. Indian J Hematol Blood Transfus. 2016;32(2):239-243. doi:10.1007/s12288-015-0553-y

6. Chehab FF, Kan YW. Detection of sickle cell anaemia mutation by colour DNA amplification. Lancet. 1990;335(8680):15-17. doi:10.1016/0140-6736(90)90138-u

7. Minkovitz CS, Grason H, Ruderman M, Casella JF. Newborn screening programs and sickle cell disease. Am J Prev Med. 2016;51(1 Suppl 1):S39-S47. doi:10.1016/j.amepre.2016.02.019

8. Screening for sickle cell disease in newborns: recommendation statementAm Fam Physician. 2008;77(9):1300-1302.

9. Manci EA, Culberson DE, Yang YM, et al. Causes of death in sickle cell disease: an autopsy study. Br J Haematol. 2003;123(2):359-365. doi:10.1046/j.1365-2141.2003.04594.x

10. Interpretation of newborn hemoglobin screening results. Michigan Department of Health & Human Services. Updated May 2015. Accessed November 16, 2021. 

11. Old J, Harteveld CL, Traeger-Synodinos J, Petrou M, Angastiniotis M, Galanello R. Haemoglobin pattern analysis. In: Prevention of Thalassaemias and Other Haemoglobin Disorders. Nicosia, Cyprus: Thalassaemia International Federation; 2012. Accessed November 16, 2021. 

12. Peters M, Fijnvandraat K, van den Tweel XW, et al. One-third of the new paediatric patients with sickle cell disease in The Netherlands are immigrants and do not benefit from neonatal screening.  Arch Dis Child. 2010;95(10):822-825. doi:10.1136/adc.2009.165290

13. Michlitsch J, Azimi M, Hoppe C, et al. Newborn screening for hemoglobinopathies in California. Pediatr Blood Cancer. 2009;52(4):486-490. doi:10.1002/pbc.21883

14. Henthorn JS, Almeida AM, Davies SC. Neonatal screening for sickle cell disorders. Br J Haematol. 2004;124(3):259-263. doi:10.1046/j.1365-2141.2003.04775.x

Reviewed by Harshi Dhingra, MD, on 11/17/2021.