Sickle Solubility Test

The sickle solubility test is a rapid screen for haemoglobin S. Red cells are lysed in a high molarity phosphate buffer containing saponin and a reducing agent (sodium dithionite). Deoxygenated HbS is insoluble in this buffer and forms a turbid suspension, while other haemoglobins stay in solution. The result is read within 5 to 10 minutes against a ruled line card.

It detects HbS but cannot separate sickle trait from sickle cell disease, and it does not identify other variants. A positive result needs confirmation by electrophoresis or HPLC.

Its main use is in services with limited laboratory capacity, and in urgent preoperative screening of patients of at risk ancestry whose status is unknown. Rapid lateral flow immunoassays for HbS are increasingly used at the point of care.

Indications. Unknown sickle status before urgent surgery or anaesthesia, acute presentation with suspected sickling when electrophoresis is delayed, and screening in settings without electrophoresis or HPLC.

Specimen. EDTA or capillary blood. The reagent must be fresh, and positive and negative controls are run with each batch.

Method. About 20 microL of blood is added to 2 mL of working reagent, mixed and left for 5 to 10 minutes at room temperature. The tube is held in front of ruled lines.

Reading.

• Positive: turbid suspension through which the ruled lines cannot be read.
• Negative: clear red solution through which the lines remain visible.

Positive. HbS trait, sickle cell disease, compound states (HbSC, sickle beta thalassaemia), and rarely other sickling variants such as HbC Harlem. The test cannot separate these, so confirmatory electrophoresis or HPLC is needed.

Negative. Excludes HbS only when the haemoglobin concentration and HbS fraction are adequate.

False negatives.

• Infants under about 6 months, because of high HbF.
• Recent transfusion or exchange transfusion.
• Severe anaemia (haemoglobin under about 70 g/L) unless packed cells are used.
• HbS under about 10 to 20% of total haemoglobin.
• Deteriorated reagent or poor mixing.

False positives. Hyperlipidaemia, raised protein concentration including paraproteins, marked leucocytosis, and polycythaemia or an excess blood volume in the reagent.

Clinical relevance. HbS trait is benign in most people, with rare complications such as haematuria and splenic infarction at altitude. Sickle cell disease carries a risk of perioperative sickling and needs specialist planning.

Critical findings needing immediate action. A positive result in a patient with pain, fever, chest symptoms or a neurological deficit needs urgent assessment for sickle crisis, acute chest syndrome or stroke, since the test cannot distinguish trait from disease.

Pitfalls.

• It is unsuitable for newborn screening.
• It cannot quantify HbS and does not detect HbC, HbD, thalassaemia or other significant variants.
• Reagent deterioration is a common cause of failure, and controls must be run.
• A negative result does not exclude sickle trait in a recently transfused patient.

Special groups. Neonates and young infants need HPLC or isoelectric focusing. In pregnancy, electrophoresis or HPLC is used instead of solubility testing. After transfusion, results are unreliable.

Follow up tests. Haemoglobin electrophoresis or HPLC, full blood count and film, reticulocyte count, DNA analysis, family testing, rapid lateral flow immunoassay for HbS.