At a Glance
Compound heterozygosity for hemoglobins D-Los Angeles and S should be suspected in a patient with known sickle trait who has a moderate anemia (7-10 g/dL), an unexpectedly severe clinical picture, or a family history of the same. The patient’s peripheral smear shows sickle cells, target cells, and increased polychromasia. This combination of hemoglobinopathies is identified reliably on Newborn Screening. Typically, one parent has known S trait with a positive sickling test, but the other does not, and, thus, the severity of the disorder in the offspring is unexpected. Family testing then reveals the hemoglobin D-Los Angeles trait or even homozygous hemoglobin D-Los Angeles in the other parent.
The carrier rate for both of these traits is frequent in areas of India (1 in 10 for hemoglobin S and 1 in 30 for hemoglobin D Los-Angeles) and less so in the African American population (1 in 13 for hemoglobin S and 1 in only 30,000 for hemoglobin D Los-Angeles), so the double heterozygote S/D combination is not that rare in these ethnic groups.
Many patients with S/D-Los Angeles disease follow a much milder clinical course than Sickle Cell Anemia, although there is considerable variability in presentation and a few patients have debilitating hemolytic disease with frequent pain crises (see chapter on Sickle Cell Anemia).
Hemoglobins D-Los Angeles and S are each benign, and have been discussed as discrete clinical entities elsewhere (see chapter on Anemia Associated with Hemoglobin D). Here, they are considered as coinherited mutations in which the interaction between the two hemoglobin species produces a significant sickling hemolytic disorder.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
The testing strategies for diagnosis and follow-up are identical to those described for Sickle Cell Anemia.(Table 1)
If the percentages of hemoglobins S and D-Los Angeles are not in the expected 50:50 split or the amount of hemoglobins A2 or F is not as expected, then further testing is indicated to rule out coinheritance of thalassemias or Hereditary Persistence of Fetal Hemoglobin (HPFH; see chapters on Anemia Associated with S-Alpha Thalassemia, Anemia Associated with Hemoglobin S-Beta Thalassemia, and Anemia Associated with Hemoglobin S-Hereditary Persistence of Fetal Hemoglobin).
In comparison to Sickle Cell Anemia, the hemolytic anemia is still moderately severe, with a hemoglobin in the range 7-10 g/dL.
|Sickling Test||Hgb Analysis||Morphology|
|positive||F < 2% A2 <3.7% S 45% D 45%||sickle cells target cells elliptocytes and spherocytes increased polychromasia|
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Factors affecting laboratory results are identical to those given for Sickle Cell Anemia.
The increased sickling in S/D disease is due to enhanced deposition of hemoglobin S polymers; the crystals contain very little hemoglobin D. Nucleation starts with hydrophobic interaction of Sß6 valine with ß85 Phenylalanine and ß88 Leucine of adjacent Sß chains. The Glutamine at Sß121 weakens the interaction between Sß116 histidine and Sß114 proline. The substitution of glutamine for glutamic acid at ß121 in hemoglobin D strengthens this attraction and facilitates both nucleation and polymerization. The resulting crystals are different in shape and size to polymers formed in Sickle Cell Anemia. Additionally, the percentage of hemoglobin S in S/D-Los Angeles disease is somewhat higher than in S trait (50 vs 40%), and, although this difference seems insignificant, it nonetheless increases the red blood cell (RBC) concentration of hemoglobin S.
Analytically, the correct identification of hemoglobin D can be challenging, since it comigrates with hemoglobin S on alkaline electrophoresis (EP) and with hemoglobin A on acid EP. The major differentials for this pattern of migration on EP is the δß hybrid hemoglobin Lepore and the thalassemic alpha chain variant G-Philadelphia (see chapters on Anemia Associated with Hemoglobin Lepore and Anemia Associated with Hemoglobin G-Philadelphia). Since both hemoglobins are under-produced relative to hemoglobins S and D-Los Angeles and additionally hemoglobin G-Philadelphia produces hybrids with beta, gamma, and delta chains (AG, FG, and G2, respectively), they are easily distinguished.
On iso-electric focusing (IEF) Hemoglobin D-Los Angeles migrates just anodally to hemoglobin S and is actually easier to discriminate when hemoglobin S is present than when it occurs with hemoglobin A.
What Lab Results Are Absolutely Confirmatory?
In practice, the demonstration of a positive Sickling test, equal sized peaks on HPLC and IEF corresponding to hemoglobin S and D-Los Angeles, with a normal A2 is considered confirmatory. The demonstration of a substitution of valine for glutamic acid in the sixth position of 1 ß-chain and substitution of glutamine for glutamic acid at position 121 of the other ß-chain is diagnostic for S/D-Los Angeles Disease. This can be confirmed by genetic testing, although the expense is rarely justified.
Many Newborn Screening programs include tests for common hemoglobinopathies and will have no difficulty identifying the presence of S/D-Los Angeles Disease. However, in the presence of such high concentrations of hemoglobin F neonatally, the Sickling test will likely be negative, especially in a pre-term infant.
Other testing modalities are as described for Sickle Cell Anemia.
What Confirmatory Tests Should I Request for My Clinical Dx? In addition, what follow-up tests might be useful?
Testing strategies for diagnosis and follow-up are identical to those described for Sickle Cell Anemia.
What Factors, If Any, Might Affect the Confirmatory Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Factors that might affect laboratory results are identical to those described for Sickle Cell Anemia.
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- At a Glance
- What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
- Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?
- What Lab Results Are Absolutely Confirmatory?
- What Confirmatory Tests Should I Request for My Clinical Dx? In addition, what follow-up tests might be useful?
- What Factors, If Any, Might Affect the Confirmatory Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?