Sun28November0440PM 33

Are the following true or false with regards to sickle cell disease?

True / False

Osteomyelitis is typically caused by E. coli bacteria Correct

The erythrocytes of haemoglobin AS patients can sickle at a PO2 of 5-6 kPa (40-50 mmHg) Correct

The Sickledex test involves adding a reagent to blood, which identifies the type of haemoglobinopathy Incorrect answer selected

The erythrocytes of haemoglobin SC patients may sickle at a PO2 of 4 kPa (30 mmHg) Correct

It is caused by the substitution of glutamic acid by valine at position 6 on the beta chain of haemoglobin Correct

Explanation

Sickle cell disease in a haemoglobinopathy caused by the substitution of glutamic acid by valine at position 6 (from the N-terminal) of the beta chain. Inherited as an autosomal gene, heterozygous (HbAS) and homozygous (HbSS) forms exist.

A low partial pressure of oxygen (PO2) causes HbS to polymerise and precipitate, resulting in sickling of the erythrocyte. HbSS patients sickle at PO2 of 5-6 kPa and HbAS patients sickle at PO2 of 2.5-4 kPa (not 5-6 kPa).

A mild disease is produced when heterozygotes for HbS combine with other haemoglobins e.g. Haemoglobin C, creating HbSC, with sickling occurring at around 4 kPa. Osteomyelitis is typically caused by unusual organisms, e.g. Salmonella.

Diagnosis of sickle cell disease requires the detection of HbS. The Sickledex test involves the addition of reagent to blood; turbidity only confirming the presence of HbS, but it gives no information on other haemoglobins. Haemoglobin electrophoresis is the only investigation that can determine the nature of the haemoglobinopathy.

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