At a Glance

Coinheritance of an α-thalassemia with homozygous SS disease should be suspected in a patient with known Sickle Anemia who has a less severe clinical presentation than expected or who has a family history of the same. These patients have a modestly elevated hemoglobin F (<20% of the total hemoglobin) in the absence of hydroxyurea moderating therapy. The presence of hemoglobins Barts and S on Newborn Screening may also prompt consideration of this combination. Hemoglobin S trait with a coinherited α-thalassemia should be considered when the percentage of hemoglobin S is less than 33%.

The α-thalassemias along with Hereditary Persistence of Fetal Hemoglobin are the main genetic moderating influences on morbidity and mortality in Sickle Anemia (see chapter on Anemia Associated with Hemoglobin S-Hereditary Persistence of Fetal Hemoglobin).α-Thalassemia results in both a decreased erythrocyte total hemoglobin content and an increased amount of hemoglobin F, which serves not only to decrease the relative percentage of hemoglobin S within the RBCs, but also to maintain a higher oxygen tension, since F is a high oxygen affinity hemoglobin. In addition, increasing the percentage of hemoglobin F induces a mild erythropoeitin-stimulated increase in red cell production, thus, partially ameliorating the anemia.

The α-thalassemias and hemoglobin S have been discussed as discrete clinical entities elsewhere. Here they are considered coinherited mutations (see chapters on Alpha Thalassemias, Sickle Cell Anemia, and Sickle Cell Trait).

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What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The tests required for diagnosis and follow-up are identical to those described for hemoglobin S. (Table 1)

Table 1
α-phenotype # of α-genes deleted Hgb F% Hgb S % in SS disease Hgb S% in AS trait
(α α/α α) 0 <2 at 2 yrs 95 35-40
α+ trait (- α/α α) 1 8 90 <33
α0 trait (- -/α α or homozygous α+ (- α/- α) 2 16 80 <30

In addition, the red cell distribution of hemoglobin F could be determined with the acid elution Kleinhauer-Betke test. Flow cytometry using fluorescently-labeled anti-hemoglobinF is increasingly being used for this purpose. A pancellular distribution of hemoglobin F is expected.

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?

It has long been known that the presentation of Sickle Anemia can vary widely and that certain geographic/ethnic similarities can be identified.

One in 13 African Americans is a carrier of the hemoglobin S mutation, and 1 in 375 has homozygous SS disease. The α3.7 (rightward) single alpha gene deletion is also very common in African Americans (30% of the population); homozygosity for this alpha deletion occurs in about 9%. This is thought to account for a significant part of the lesser severity of presentation than in Africa. The cis 2 gene deletion (α0 trait) is rare in African Americans but is prevalent in Southeastern Asians. Another α-gene deletion results from a single mutation near the 3′ end of the α2 gene and is thought to be present in up to 50% of Saudi patients with Sickle Anemia. A similar incidence of α-thalassemia occurs in India, but, again, the mutation is different (homozygous leftward deletion, – α/- α).

The combination of α-thalassemia and Sickle Anemia is, therefore, to be expected, and a 2 gene deletion affords the most benefit. These patients show less vaso-occlusion and ischemic reperfusion injury and have less hemolytic anemia and, consequentially, reduced vasculopathy and cholelithiasis. However, there is little improvement in acute pain crises (and indeed may even be an increase) or in Acute Chest Syndrome.

Although HPFH also has an ameliorating effect on Sickle Anemia, its contribution is thought to be much less impressive, so it is important to determine the correct cause of the elevated hemoglobin F. It is important to arrive at the correct diagnosis for both counseling and treatment options.

The distribution of hemoglobin F among the red blood cells (RBCs) is pancellular in α-thalassemia, but, since it is only rarely heterocellular in HPFH, performing a test to determine the distribution pattern may not contribute to the diagnosis. Family studies can be informative in this regard and may negate the need for the expense of gene analysis.

Other causes of elevated hemoglobin F should be considered:

Hydroxyurea therapy is widely used in treatment of Sickle Anemia to boost the production of hemoglobin F. This is important, since hemoglobin S only denatures in its deoxygenated state. However, the drug is also given in polycythemia, and, as chemotherapeutic agent, if the drug history is unknown, an elevated hemoglobin F should be interpreted with caution.

In patients without Sickle Anemia, hemoglobin F concentrations are expected to decline to adult concentrations by 6 months; in Sickle Anemia this process takes about 2 years.

Transfused blood is always assumed to be 95% hemoglobin A, with less than 2% hemoglobin F, but, since HPFH is often silent, there is a possibility that a transfused unit could contain a significant percentage of hemoglobin F. A transient rise in F after transfusion is indicative of this.

What Lab Results Are Absolutely Confirmatory?

Many Newborn Screening programs include tests for common hemoglobinopathies. The hemoglobin S disease will be readily identified and can be differentiated from S Trait. However, a single gene α-thalassemia will not be detected at this stage.

The percentage of hemoglobin Barts (γ-chain tetramers) can be informative of the number of alpha genes deleted the same way hemoglobin F can be in adults. A 2 α-gene deletion will be indicated by the presence of approximately 5-15% hemoglobin Barts, but this will decline as the fetal hemoglobin decreases during the first 6 months of life. It is not replaced by hemoglobin H. The presence of larger amounts of hemoglobin Barts on Newborn Screening (20-30%) indicates a 3-gene deletion, which becomes replaced by hemoglobin H (ß-tetramers). Hemoglobin H disease has unpredictable presentation when it rarely is seen with hemoglobin S.

It is, therefore, important to follow the percentage of hemoglobin F, and, after 2 years of age, the diagnosis may need to be reassessed.

The other tests required for diagnosis and follow-up are identical to those described for hemoglobin S.

What Confirmatory Tests Should I Request for My Clinical Dx? In addition, what follow-up tests might be useful?

The tests required for diagnosis and follow-up are identical to those described for hemoglobin S.

In patients who have different percentages of hemoglobins S, A, and F to those expected in uncomplicated presentations, considerations of alternative hemoglobin variants are important. Even with an α-thalassemia in S trait, there is sufficient hemoglobin present to give a positive Sickling test. Other hemoglobins that also give positive Sickling tests may need to be considered. With the exception of C-Harlem, most of these are rare or isolated reports. With the exception of Porto-Alegre, all contain the S mutation (ß6 Glu → Val) in addition to the mutation subsequently shown:

Hgb C-Harlem (ß73 Asp → Asn) (C-Georgetown)

Hgb C-Ziquinchor (ß58 Pro → Arg)

Hgb S-Oman (ß121 Glu → Lys)

Hgb S-Providence (ß82 Asn → Asp)

Hgb S-Travis (ß121 Ala → Val)

Hgb Jamaica Plain (ß68 Leu → Phe)

Hgb Antilles (ß23 Val → Ile)

Hgb Porto-Alegre contains only the (ß9 Ser → Cys) mutation

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?

The factors that affect laboratory tests are identical to those described for hemoglobin S.