NEWBORN SCREENING In the United States, most states with a sizable African-American population have implemented newborn screening programs to identify affected individuals at birth. Success in preventing early complications and assuring reliable follow-up mechanisms has had a significant effect on reducing overall mortality from the disease. Patients identified through programs such as these are then enrolled for comprehensive treatment. Cord blood can be utilized right in the delivery room.
Hemoglobin electrophoresis is the "gold standard" and most accurate diagnostic study to determine and distinguish sickle cell anemia (homozygous Ss), sickle cell trait (HgbSC), and sickle cell thalassemia. In the newborn and infant, this test is always necessary to confirm the diagnosis.
LABORATORY STUDIES As always, prudent laboratory testing is tailored to the clinical signs and symptoms of patients as they present to the ED. The following is a list of some commonly ordered diagnostic studies.
SICKLE PREP If the diagnosis is in question or in previously undiagnosed cases, the emergency physician will need a rapid assay for determining the presence of SCD. The sickle prep is a peripheral blood smear specially prepared so as to induce RBC sickling. These cells are then seen on routine microscopy of the smear. Tests such as the Sickledex and Metabisulfite tests have been used. While these tests can be useful in the ED, they are limited in their ability to diagnose SCD in infants under the age of 4 to 6 months, and they cannot diagnose sickle cell trait.
COMPLETE BLOOD COUNT (CBC) All patients who present to the ED in sickle cell crisis or any other complaint referable to SCD should have a CBC performed; of paramount importance is the serum hemoglobin level. The average hemoglobin in patients with SCD is generally between 6 and 9 g/dL. Obtaining a prior baseline level is critical, because any patient with a drop of 2 to 3 g/dL in hemoglobin or a decline in the serum hematocrit by 4 to 6 percent may reflect severe hematologic crisis or hypovolemia and shock. Platelet counts are usually elevated at baseline. White blood cell (WBC) counts are also often elevated at baseline, ranging from 12,000 to 18,000 pL, usually with a normal differential. Any rise in the total WBCs or leftward shift may indicate the presence of significant underlying pathology. The peripheral smear often shows sickled cells as well as Howell-Jolly bodies, indicative of splenic dysfunction and failure.
RETICULOCYTE COUNT Because of accelerated RBC turnover, reticulocytes appear in greater numbers on the peripheral smear. The mean reticulocyte count is approximately 12 percent (with a range of 5 to 15 percen) in SCD patients. With a reticulocyte count of 3 percent or less, aplastic crisis should be considered. An extremely high count may be indicative of hemolysis. Certainly any decrease of 2 g/dL or greater in serum hemoglobin levels warrants obtaining a reticulocyte count.
BLOOD CULTURES Blood cultures should be obtained in any SCD patient with a temperature higher than 38.4°C (101°F) and in any patient suspected of having bacteremia or sepsis. Any child with radiographic evidence of infiltrative disease should also have blood cultures drawn.
URINALYSIS (UA) Any patient with a fever and/or urinary symptoms (especially dysuria or hematuria) should have a UA (and appropriate culture) obtained. RBCs or tissue in the urine are seen with papillary necrosis/renal crisis. Isosthenuria, the inability to concentrate urine, is typically present, with specific gravities of about 1.010. The specific gravity can be quite low in patients with SCD even when they are dehydrated.
ELECTROLYTES Electrolytes should be obtained in any SCD patient who appears clinically to be dehydrated. Significant imbalances can occur because of isosthenuria.
ARTERIAL BLOOD GAS (ABG) An ABG should be obtained in all SCD patients with chest pain not typical of vasoocclusive crisis or in patients with any respiratory complaints. A large arterioalveolar (A-a) gradient suggests either pulmonary infection, infarction, or potential thromboembolus. It is helpful to have a baseline reference ABG since most adolescents and adults with SCD exhibit a mild to moderate chronic hypoxia. Generally, arterial P O2 levels less than 60 mmHg in adults or 70 mmHg in children are indicative of acute pathology.
LIVER ENZYMES Liver enzymes may be helpful in SCD patients who present with abdominal pain. However, baseline abnormalities are common, especially an elevated indirect bilirubin, which is usually secondary to the chronic hemolytic state of SCD.
CEREBROSPINAL FLUID ANALYSIS A lumbar puncture should be performed when meningitis is a consideration.
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