Required section · Section 3 of 6

Focused instruction: pair methods and name limits

Cation-exchange HPLC separates fractions by retention time and can quantify HbA2 and HbF. Capillary electrophoresis separates by electrophoretic mobility and displays zones, giving a complementary principle. Retention-time and zone identifications remain method-specific. Review the analyzer, reagent application, calibrator, controls, software, and reportable fractions on the local method card before releasing an interpretive comment.

An S-position band on alkaline electrophoresis is not uniquely HbS because HbD, HbG, and Hb Lepore can co-migrate there. Acid electrophoresis can separate HbS from HbD and HbG in that setting. A major fraction in an S zone on capillary electrophoresis supports a sickling hemoglobin but does not genotype it. Molecular HBB testing or family studies can resolve remaining ambiguity under the local pathway.

A sickling-solubility screen observes turbidity when reduced sickling hemoglobin becomes insoluble in concentrated phosphate buffer. It is qualitative and does not distinguish sickle cell disease from trait. The SICKLEDEX IFU, document 350430-27 revision 2024-05, states a kit-specific 15% HbS detection limit at total hemoglobin 12 g/dL and cautions that high HbF below age six months and recent transfusion can affect results. Use solubility as a limited screen, never as the genotype result.

Retention-window HPLC chromatogram shows baseline, labeled HbF, HbA, HbA2, and S-window retention windows in cation-exchange elution order and integrated peak areas beside a capillary electrophoresis zone schematic. The label states that an S window does not establish genotype. — Figure 1Retention-window HPLC chromatogram with baseline, expected fraction peaks, retention windows, and integrated S-window; paired with a capillary-electrophoresis schematic.

Chromatogram retention windows and reported areas.
Window	Retention range (min)	Reported area
HbF	0.9–1.1	12.0%
HbA	1.2–1.4	31.0%
HbA2	1.5–1.7	3.0%
S	1.8–2.1	54.0% (integrated)

Knowledge checks

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Knowledge check 1

An alkaline electrophoresis pattern has an S-position band. What is the most appropriate next analytic step when identification remains unresolved?

Choose one option.

Report HbS as final because the band is in the S position.

Use acid electrophoresis or another complementary validated method.

Use solubility alone to establish genotype.

Knowledge check 2

Which limitations apply to the stated SICKLEDEX solubility method? Select all that apply.

Choose at least 3 options.

It is a qualitative turbidity screen.

High HbF can cause false-negative results below age six months.

Recent transfusion can affect the result.

A positive result distinguishes trait from disease.

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