Diagnostic value of anemia, red blood cell morphology, and reticulocyte count for sickle cell disease

ORIGINAL CONTRIBUTION

anemia; sickle cell disease

D a g n o s t i c Value of Anemia, Red Blood Cell Morphology, and Reticulocyte Count for Sickle Cell Disease From the Departments of Pediatrics* and Anesthesiology, t Medical College of Wisconsin,

Joseph D Losek, MD, FACEP* Thomas R Hellmich, MD* George M Hoffman, MD ~

Children's Hospital of

Study objective: Todetermine the diagnostic value of anemia, RBC morphology, and reticulocyte count for differentiating patients with sickle cell trait from those with sickle cell disease, who have acute medical or surgical conditions and a positive sickle cerl screen.

Wisconsin, Milwaukee.

Design: Retrospective chart review.

Received for publication

S e t t i n g : A midwest urban children's hospital with 220 beds and 36,000 emergency department visits per year.

October 18, 1991. Revision received February 11, 1992. Acceptedfor publication February 25, 1992.

Participants: One hundred six patients with sickle cell trait and 152 patients with sickle cell hemoglobinopathies. Results: Anemia was observed significantly more often in patients with sickle cell disease compared with sickle cell trait (P< .001) at all ages 3 months and older. However, anemia alone as a diagnostic test lacked high sensitivity and specificity in children less than 4 years old. Sensitivity approached 100% with the presence of anemia, abnormal RBC morphology, or reticulocyte count of more than 2%. Conclusion: Absence of anemia alone does not exclude the diagnosis of sickle cell disease in children tess than 4 years old. To differentiate trait from sickle cell disease, we recommend determination of not only hemoglobin adjusted for age but also of RBC morphology and reticulocyte count on all children presenting with acute medical and surgical conditions and a positive sickle cell screen. [Losek JD, Hellmich TR, Hoffman GM: Diagnostic value of anemia, red blood cell morphology, and reticulocyte count for sickle cell disease. Ann EmergMedAugust 1992;21:915-918.]

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S I C K L E CELL D I S E A S E

Losek, Hellmich & Hoffman

INTRODUCTION Children with sickle cell disease are at increased risk for sudden life-threatening complications such as sepsis, splenic sequestration, tissue hypoxemia, and accelerated vaso-occlusion during hypermetabolic stress. Children with sickle cell trait are not at risk for these complications. 1 Therefore, it is necessary to determine the sickle cell status of black children presenting to emergency departments with fever or abdominal pain or of those requiring surgery. 2 The genotypic diagnosis of sickling hemoglobinopathies is determined by hemoglobin electrophoresis, a test that is time consuming and not continuously available. Despite the recent development of newborn screening for sickle cell disease, parents commonly do not know their child's sickle cell status and do not seek appropriate follow-up care. 3,4 A rapid and readily available sickle cell screen can be performed to aid in the acute management of these children.5, 6 However, this test is positive not only for sickle cell hemoglobinopathies but also for sickle cell trait. Sickle cell trait occurs in 8.5% of black children, in whom the disease occurs in one of 500. 5 Thus, the sickle cell screening test lacks specificity to identify a population at risk for life-threatening complications, and it is common practice to hospitalize and treat the febrile black child with a positive sickle cell screening test pending hemoglobin electrophoresis results. 3 Likewise, elective surgery may be needlessly delayed, or a simple or exchange blood transfusion may be performed before nonelective surgical procedures. Such therapy may be invasive and expensive and have potentially serious complications. Therefore, we conducted a study to determine the diagnostic value of anemia, RBC morphology, and reticulocyte count for differentiating patients with sickle cell trait from those with sickle cell disease.

MATERIALS AND METHODS The laboratory log at Children's Hospital of Wisconsin was used to identify the sickle cell disease and trait populations. The medical records of 139 patients with sickle cell trait and of 199 patients with sickle cell disease obtained J a n u a r y through December 1988 were reviewed. The following data were recorded: age, sex, results of CBC, reticulocyte count, and hemoglobin electrophoresis. For the patients with sickle cell trait, temperature, indication for sickle cell screen, toxic appearance, and hospitalization also were recorded. All patients with sickling hemoglobinopathies identified by electrophoresis and with CBC and RBC morphology were

Age Less than 3 months 3 to 5 Months 6 to 23 Months 2to 11 Years 12 Years and older Female Male

30/916

Hemoglobin for Anemia ( g / d L ) < 9.5 < 10.0 < 10.5 < 11.0 < 12.0 < 13.0

Table 1.

Definition of anemia by age category

included in the sickle cell disease population. All patients with positive sickle cell screen (differential solubility test for hemoglobin S) but electrophoresis negative for hemoglobinopathies, CBC, and RBC morphology were included in the trait population. We thus identified 106 sickle cell trait patients and 152 sickle cell disease patients for analysis. Absence of reticulocyte count was not used to exclude patients. Anemia was defined according to age and hemoglobin (Table 1). 7 Abnormal RBC morphology was graded with a numerical value of I (slight or few), 2 (moderate), or 3 (marked or many) according to the percentage of RBCs with a given morphology (Table 2). These gradings were made retrospectively on the basis of the patient's laboratory report. Differences between groups were analyzed by Z2. Differences were considered significant at P < .05. Differences in prevalence of anemia, high reticulocyte count, and abnormal RBC morphology were used to construct the sensitivity and specificity. RESULTS The sickle cell trait population comprised 106 patients aged 2 months to 16 years, and the disease population comprised 152 patients aged 2 months to 21 years. The median ages were not significantly different per the Wilcoxon rank-sum test (P = .06). There were 37 female and 69 male patients in the trait population and 68 female and 84 male patients in the sickle cell disease population. The sex distributions of the two populations were not different (P =. 114 by X2). In the sickle cell trait population, a sickle cell screen was performed because of fever (62), impending surgery (19), routine screening (11), family history of sickle cell disease (five), extremity pain (one), abdominal pain (one), stroke (one), failure to thrive (one), and abdominal mass (one). Of the 62 patients with fever and positive sickle cell screen, 46 (74%) were admitted for antibiotic therapy pending blood culture or hemoglobin electrophoresis results. Twenty-nine Table 2.

Grading of RBC morphology Grade Morphology Type Anisocytosis* Poikilocytosis t Polychromasia Elliptocyte Target Tear drop Schistocytes* Sickle ceil Howell-Jolly bodies

1-Slight or Few (%)

2-Moderate (%)

3-Marked or Many (%)

5-25 5-25 1-3 5-15 5-15 2-5 2-5 2-5 I-3

26-75 26-75 4-25 16-75 16-75 6-40 6-30 6-40 4-0

> 75 > 75 > 25 > 75 > 75 > 40 > 30 > 40 > 10

Abnormal RBCmorphology:Presenceof sickledcellsor Howell-Jollybodies,or grade2 or 3 elliptocytosis,schistocytosia,targets,tear drops,polychromasia,anisocytosis,or poikilocytosis. *Vafiance in sizein that cell population,not fromthe normal. *Variancefrom normalroundshape. *Includes helmets,triangle cells,and anyfragmentedcell.

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SICKLE CELL D I S E A S E Losek, Hellmich & Hoffman

(63%) of these febrile children were described as nontoxic, bat admission was considered necessary only because of the suspicion of sickle cell disease. Anemia, as defined in Table 1, was present significantly more often in sickle cell disease patients compared with trait patients at all ages 3 months and older (Table 3). However, the diagnostic value of anemia alone, as characterized by both a high sensitivity and high specificity, was only apparent at the age of 4 years and older (Table 4). To increase the diagnostic yield, we also studied reticulocyte count and RBC morphology characteristics for their probability in differentiating trait from disease (Table 5). Reticulocyte count of more than 2% had a sensitivity of 93%. Anemia or reticulocyte count of more than 2% and anemia or abnormal RBC morphology had sensitivities of 97%. The sensitivity was the highest (99%) when anemia, abnormal RBC morphology, and reticulocyte count of more than 2% were combined. Highly specific (> 95%) tests for excluding sickle cell disease are abnormal RBC morphology, sickled cells, and Howell-Jolly bodies. Eleven (10.7%) of the sickle cell disease patients less than 4 years old were not anemic. The genotypic diagnosis of these patients was six with SS hemoglobin, three with SC hemoglobin, and two with S ~-thalassemia hemoglobin. Four had normal RBC morphology, and one had a reticulocyte count of less than 2%. This patient was a 2-month-old with 69% fetal hemoglobin. DISCUSSION The leading cause for death in children less than 3 years old with sickle cell hemoglobinopathies is serious bacterial infecti0n.l,a Early recognition of sickle cell disease and prophyTable3. Prevalence of anemia by age and sickle cell disease stat~ Trait Disease Age Patients Patients Lessthan 3 months 3to 5 Months 8 to 11 Months 12to 23 Months 2to 3 Years 4 to 11 Yea rs 12to 20 Years

0/2 1/11 9/31 4/21 5/18 2/19 0/4

P*

1/2 11/14 32/34 22/24 26/29 84/36 13/13

248 .001 .001 .001 .001 .001 .001

*Probabilitythat disease patientshavesame incidence of anemia as trait patients, by X2

No.

Less than 3 months 3 to 5 Months 6 to 11 Months 12to 23 Months 2 to 3 Years 4 to 11 Years 12to 20 Years

4 25 65 45 47 55 17

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Sensitivity (%) 50 79 94 92 90 94 100

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Table 5. Diagnostic value of anemia, RBC morphology, and reticulocyte count Test (N = 258 unless stated)

Table 4. Diagnostic value of anemia alone at different ages Age

lactic antibiotic therapy have decreased the mortality rate. 9-11 For children less than 5 years old with sickle cell disease who present with fever, hospitalization for IV antibiotics is recommended. 12 For these reasons, young febrile black children with unknown sickle cell disease status are commonly screened for sickle cell disease in pediatric EDs. 3 When sickle cell screens are positive, important therapeutic decisions must be made without the benefit of absolute genotypic diagnosis: to admit a febrile infant and treat with antibiotics, to cancel elective surgery, or to perform simple or exchange transfusion before nonelective surgical procedures. Although the conservative course might be to assume that a patient has sickle cell disease if the screening test is positive, these therapies cannot be accepted without risk, inconvenience, or expense. For instance, a febrile but nontoxic-appearing 2-year-old with a low probability of disease might be better served by close outpatient follow-up than by 48 to 72 hours of hospitalization and parenteral antibiotic therapy. Likewise, performing an exchange transfusion before urgent surgery in a patient with a low, but nonzero statistical risk of having a sickling hemoglobinopathy may place the patient at higher risk of bad outcome than would foregoing the transfusion. Screening tests for a potentially life-threatening illness must have a high sensitivity. Using hemoglobin alone to differentiate trait versus sickle cell disease in young children with positive sickle cell screening tests is not recommended. In our study, the sensitivity of anemia was only 91%. Eleven (10.7%) of the sickle cell disease patients less than 4 years old were not anemic. Therefore, the assessment of the young febrile black child with unknown sickle cell status must be determined by a sickle cell screening test and not by a CBC. The absence of anemia does not exclude sickle ceil disease. The sensitivity of sickle cells and Howell-Jolly bodies was low. Howell-Jolly bodies are small, round or oval structures, pinkish or bluish in color, observed in RBCs in anemia, leukemia, and asplenia. Therefore, a careful study of the peripheral smear is necessary. The absence of sickle cells or Howell-Jolly bodies does not exclude sickle cell disease. ~ suspicion of sickle cell disease should be made if there are more than 5% schistocytes or tear drop cells, more than 15%

Specificity (%) 100 91 71 81 72 89 100

Anemia* Abnormal RBC Sickle cells Howell-Jolly bodies High retics* (N = 152) Anemia or high retics (N = 152) Anemia or abnormal RBC Anemia, abnormal RBC, or high retics (N = 152)

Sensitivity (%)

Specificity (%)

91 88 56 22 93

81 95 100 100 85

97 97

62 77

99

54

* Age-adjusted anemia. t Reticulocytecount> 2.0%.

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Loseko HeUmich & Hoffman

target or elliptocyte cells, and more than 25% anisocytosis (variance in cell size) or poikilocytosis (variance in r o u n d shape). However, the sensitivity of a b n o r m a l RBC morphology alone was only 88%. Compared with anemia and abnormal RBC morphology, high reticulocyte count (> 2%) alone h a d the greatest sensitivity (93%). The highest sensitivity (99%) was achieved when anemia, abnormal RBC morphology, or high reticulocyte count was present. Therefore, because admission for IV antibiotics is recommended for all febrile children less than 5 years old with sickle cell disease, 12 we recommend performing a sickle cell screening test on young febrile black children whose sickle cell status is unknown. If the screening test is positive, our study found that children with sickle ceil disease will have a 99% probability of having anemia, abnormal RBC morphology, or high reticulocyte count. When interpreting the specificity of the study variables, it is important to realize that the disease-free group in our study was defined as patients with sickle cell trait. The incidence of study variables (ie, anemia, RBC morphology, and reticulocyte) may be different if the disease-free group included patients not only without sickle cell disease but also without sickle cell trait. However, our disease-free group was from patients with a positive sickle cell screening test. Because we recommend performing a sickle cell screening test on young febrile black children with unknown sickle cell disease status to determine those children at risk for sickle cell disease, we believe the specificities r e p o r t e d are not misleading. It is important to comment on the 2-month-old sickle cell patient who was not anemic and had a normal reticulocyte count and RBC morphology. Splenic dysfunction develops as fetal hemoglobin decreases. 13-14 The onset of functional asplenia has been shown to correlate with a decrease in fetal hemoglobin level less than 20%. This patient h a d a fetal hemoglobin of 69%. Therefore, the sickle ceil patient less than 3 months 01d who has normal hemoglobin concentration, RBC morphology, and reticulocyte count is likely to have a high fetal hemoglobin level and not be at relatively increased risk for infection.

REFERENCES 1. Platt os, Nathan D6: Sickle cell disease, in Nathan DG, Oski FA (eds): Hematologyof Infancy and Childhood,ed 3. Philadelphia, WB Saunders, 1987, p 655-698. 2. Schiffman MA: Preventable sudden death in children with sickle,hemoglobinopathies and fever: The need for a protocolizad approach. Ann Emerg Med 1991;29:1043-1044. 3. Losek JL: Sickle cell screening practice in pediatric emergency departments. Pediatr Emerg Care 1991;7:278-280. 4. Miller ST, Stilerman TV, Rao SP, et ah Newborn screening for sickle cell disease. When is an infant 'lost to follow-up'? Am J Ois Child 1990;144:1343-1345. 5. Greenberg MS, Harvey HA, Morgan C: A simple and inexpensive screening test for sickle hemoglobin. N Engl J Meal 1972;286:1143-1144. 6. Ral[ard MS, Radel E, Sakhadeo S, et ah A new diagnostic test for hemoglobin S. J Pediatr 1970;78:117-119. 7. Moll MM: Hematology, in 6reene M6 (ed): The Harriet Lane Handbook, ed 12. St Louis, Mosby Year Book, 1991, p 42. 8. Leikin SL, 6allagher D, Kinney TR, et ah Mortality in children and adolescents with sickle cell disease, Pediatrics 1989;84:500-508. 9. Vichinsky E, Hurst D, Earles A, et ah Newborn screening for sickle cell disease: Effect on mortality. Pediatrics 1988;81:749-755. 10.6aston MH, Verter JI, Woods G, at al: Prophylaxis with oral penicillin in children with sickle cell anemia: A randomized trial. NEnglJMed1986;314:1593-1599. 11. Pewars B, 0verturf GD, Weiss J, et ah Pneumococcal septicemia in children with sickle cell anemia: Changing trend of survival. JAMA 1981;245:1839-1842. 12. Vichinsky E: Sickle cell disease, in Gegis S, Kagan B (eds):Pediatric Therapy, ed 12. Philadelphia, WB Saunders,1988. 13.0'Brien RT, Mclntosh S, Aspnes 6T, et ah Prospective study of sickle cell anemia in infancy. J Pediatr 1976;89:205-210. 14. Rogers BW, Serjeant BE, Sergeant GR: Early rise in "pitted" red cell count as a guide to susceptibility to infection in childhood sickle cell anemia. Arch Ois Child1982;57:338-342.

Address for reprints: Joseph D Losek, MD, FACEP, Children's Hospital of Wisconsin, 9000 West Wisconsin, MS 756, Milwaukee, Wisconsin 53226.

CONCLUSION

The presence of anemia alone does not differentiate trait versus sickle cell disease in young children with a positive sickle cell screening test. The probability of making the correct diagnosis is greatly enhanced when the RBC morphology and reticulocyte count are also analyzed. Therefore, we recommend determining hemoglobin adjusted for age, RBC morphology, and reticulocyte count on all children presenting with acute medical or surgical conditions and a positive sickle ceil screen. The authors thank Frances Sommer for her assistance in the preparation of this manuscript.

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