Sickle Screen

Background

Haemoglobin S (HbS) or sickle cell haemoglobin, results from an autosomal recessive mutation to the beta globin gene. Red cells containing HbS have a reduced deformability, under conditions of Hypoxia they form the so-called sickle shape and are easily destroyed resulting in occlusion of the microcirculation and a chronic haemolytic anaemia. Heterozygotes have sickle cell carrier status, homozygotes have sickle cell disease. HbS may be found in conjunction with other Haemoglobin mutations and compound heterozygotes may display the features of homozygous disease or be asymptomatic depending on the combination. The screening test will detect the presence of HbS in a sample, it will not distinguish between HbAS (heterozygote) and HbSS (homozygote), positive samples are sent for Haemoglobin Quantitation. The test is used as a quick pre-op screening test in patients with a family history of HbS or if the patient is from a region with a high prevalence of the mutation (Africa, Middle East, Mediterranean and India) as general anaesthesia presents a significant risk to patients with this mutation.

Sample requirements

• Whole blood collected into 1x vacutainer containing EDTA (purple top)
• A full blood count (FBC) and ferritin is also required for interpretation
• Please indicate the patient’s ethnicity/country of origin in the clinical details
• If the test is part of the Sickle Cell and Thalassaemia screening programme, all samples must be accompanied with a family origin questionnaire

Minimum volume

• Adult: 2ml
• Paediatric : 1ml

en handling all blood samples but special precautions will be taken when dealing with High Risk. These samples should be marked with High risk stickers and the clinical details stated on the request form.

Known CJD and query CJD patients will only be analysed after agreement with the Consultant Haematologist who will then advise the laboratory of which tests are required.

Interpretation

• Positive – Suggest Haemoglobin S present
• Negative – Suggest Haemoglobin S not present
• All results require confirmation via High Performance Liquid Chromatography via a Haemoglobinopathy screen

Limitations

Due to the diversity of Haemoglobin variants and thalassaemia syndromes there will always be some situations that require further tests, or family studies before a useful clinical diagnosis can be achieved.

If an individual has had a blood transfusion and any of the transfused red cells are still present, misleading data and conclusions may result. In this situation, repeat testing should occur at least 4 months after the transfusion.

Other clinical situations that may prevent accurate interpretation of the results are HIV, severe iron deficiency anaemia (including a Hb level of <80g/L), liver disease, and B12/folate deficiency.

Where a patient has had a bone marrow transplant (BMT) or gene therapy it is likely that the results obtained will reflect the BMT donor / post gene therapy status and will not accurately represent the genetic status of the patient.

It may not be possible to provide accurate results on short, clotted or haemolysed samples. These will be rejected with an appropriate comment.

Samples must not be exposed to extreme temperature e.g. next to heat source, left in direct sunlight or frozen.

Analysing laboratory

Haematology laboratory, The James Cook University Hospital, Marton Road, Middlesbrough, TS4 3BW