A patient shows a negative dithionite solubility test and electrophoresis with bands in the A and S regions on cellulose acetate at pH 8.6; on citrate agar there is only a band in the A region. Which hemoglobin variant is compatible with these results?

A patient shows a negative dithionite solubility test and electrophoresis with bands in the A and S regions on cellulose acetate at pH 8.6; on citrate agar there is only a band in the A region. Which hemoglobin variant is compatible with these results?

• A. Hb A D
• B. Hb A S
• C. Hb A A
• D. Hb S S

Answer: (A)

Understanding how hemoglobin variants separate by charge helps you distinguish them across different media. At alkaline pH on cellulose acetate, Hb A migrates to the A zone, Hb S to the S zone, and Hb D co-migrates with Hb S in that same S zone. The dithionite solubility test detects Hb S-containing hemoglobins; a negative result means Hb S is not present. In this case, you see a band in the A zone and a band in the S zone on alkaline electrophoresis, but the dithionite test is negative, so the S-zone band is not Hb S. That pattern fits Hb D, which migrates with Hb S in the alkaline system but is not S. On citrate agar, Hb D migrates with Hb A, so you would expect just a single band in the A region. The observed citrate agar result—only an A-region band—matches this, reinforcing Hb A D as the underlying variant.

Understanding how hemoglobin variants separate by charge helps you distinguish them across different media. At alkaline pH on cellulose acetate, Hb A migrates to the A zone, Hb S to the S zone, and Hb D co-migrates with Hb S in that same S zone. The dithionite solubility test detects Hb S-containing hemoglobins; a negative result means Hb S is not present. In this case, you see a band in the A zone and a band in the S zone on alkaline electrophoresis, but the dithionite test is negative, so the S-zone band is not Hb S. That pattern fits Hb D, which migrates with Hb S in the alkaline system but is not S.

On citrate agar, Hb D migrates with Hb A, so you would expect just a single band in the A region. The observed citrate agar result—only an A-region band—matches this, reinforcing Hb A D as the underlying variant.