The diagnosis of SCD is usually established in early life based on the family history, ideally before the child begins to develop complications. The diagnosis is made by documenting the presence of sickle hemoglobin. Sickled red blood cells should be seen on the peripheral smear. Quick tests to determine the presence of sickle hemoglobin are the metabisulfite test and the Sickledex (Ortho-Clinical Diagnostics) test. Hemoglobin electrophoresis is required to distinguish homozygous SS disease from sickle cell trait and the other heterozygous sickle syndromes.
LABORATORY STUDIES The laboratory evaluation required for an individual patient who presents to the ED can be tailored to the symptoms of the patient and the physical findings. Commonly obtained studies include the following evaluations.
Complete Blood Count A complete blood count (CBC) should be obtained in most sickle cell patients with severe crisis. The "normal" white blood cell count in patients with SCD ranges from 12,000/pL to 18,000/pL but usually has a normal differential. Elevations higher than the patient's baseline or the presence of a left shift are notable. The baseline hemoglobin in sickle cell patients is typically 6 to 9 g/dL. A fall in the hemoglobin by 2 g/dL or a fall in the hematocrit by 4 to 6 percent may reflect hematologic crisis or blood loss. The platelet count is often elevated at baseline. The peripheral smear should show sickled cells and Howell-Jolly bodies, which indicate the loss of splenic function.
Reticulocyte Count A reticulocyte count should be obtained if the hemoglobin has fallen by 2 g/dL or more. The baseline reticulocyte count is typically 5 to 15 percent; lower values may reflect aplastic crisis.
Electrolytes Electrolytes should be obtained in any patient who appears significantly dehydrated, because imbalances occur readily due to isosthenuria.
Liver Function Tests Liver function tests should be obtained in sickle cell patients who present with abdominal pain. These patients often have baseline abnormalities and a chronically elevated indirect bilirubin due to chronic hemolysis.
Erythrocyte Sedimentation Rate The erythrocyte sedimentation rate (ESR) is not a reliable indicator of inflammation or infection in patients with SCD because sickled cells cannot form rouleaux, which is the basis for the test. The ESR is helpful only if it is elevated, since a normal value does not rule out infection or inflammation.
Arterial Blood Gas An arterial blood gas should be obtained in any patient with SCD who presents with respiratory complaints. It is very helpful to know a baseline value because adults may have chronic mild to moderate hypoxia. A Pao2 of less than 60 mmHg in adults or less than 70 mmHg in children usually indicates an acute problem.
Urinalysis A urinalysis should be obtained in any sickle cell patient with urinary symptoms or fever.
RADIOLOGIC STUDIES Chest X-ray A chest x-ray should be obtained in any patient with pulmonary signs or symptoms. In the acute chest syndrome, the chest x-ray may be normal for 2 days, so a normal radiograph does not exclude a problem.
Bone Films Bone films should be considered for the patient who presents with localized bone tenderness. Plain radiographs will not show infarction and will only show osteomyelitis after about 10 days. A bone scan or MRI can show changes of osteomyelitis within 24 h.
Abdominal Ultrasound or CT Scan An abdominal ultrasound or CT scan may be helpful in patients with abdominal pain. The presence of vasoocclusive crisis will not alter these studies; however, they may show changes of cholecystitis, pancreatitis, bowel infarction, abscesses, or appendicitis.
Computed Tomography of Head or MRI Scan Computed tomography of the head (head CT) or an MRI scan should be obtained in any sickle cell patient who presents with neurologic symptoms or signs. MRI is a more sensitive study in this setting.
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