BACKGROUND AND OBJECTIVES

The neonatal hereditary spherocytosis (HS) index, defined as the mean corpuscular hemoglobin concentration divided by the mean corpuscular volume, has been proposed as a screening tool for HS in neonates. In a population of mostly white infants, an HS Index >0.36 was 97% sensitive and >99% specific. We evaluated the utility of the HS Index among a more racially and ethnically diverse population and determined if its discrimination varies with total serum bilirubin (TSB) levels.

METHODS

Infants born at ≥35 weeks’ gestation at 15 Kaiser Permanente Northern California hospitals from 1995 to 2015 were eligible (N = 670 272). Erythrocyte indices from the first complete blood count drawn at ≤7 days and TSB levels drawn at ≤30 days were obtained. Diagnoses of HS were confirmed via chart review.

RESULTS

HS was confirmed in 79 infants, 1.2 per 10 000. HS was more common among infants of white and “other” race or ethnicity and among those with higher peak TSB levels. The area under the receiver operating characteristic curve for the HS Index was 0.84 (95% confidence interval 0.78–0.90). Likelihood ratios ranged from 10.1 for an HS Index ≥0.380 to 0.1 for an HS Index <0.310. Dichotomized at 0.36, the HS Index was 56% sensitive and 93% specific. Discrimination of the HS Index appeared best among infants with TSB levels <10 mg/dL.

CONCLUSIONS

The HS Index, when obtained from a CBC drawn within the first week after birth, had only modest ability to alter the probability of HS.

What’s Known on This Subject:

Hereditary spherocytosis (HS) frequently presents with neonatal hyperbilirubinemia but is often diagnosed much later. The Neonatal HS Index, obtained from erythrocyte indices, may be a rapid and inexpensive screening tool for HS in neonates.

What This Study Adds:

The HS Index is limited by the low prevalence of HS but, given its minimal cost, may be helpful in situations in which even a low post-test probability would prompt more aggressive phototherapy of problematic jaundice while pursuing additional workup.

The diagnostic evaluation of newborns with significant or prolonged neonatal hyperbilirubinemia that does not respond to phototherapy as anticipated can be challenging. Other than alloimmune hemolytic disease, typically diagnosed with a direct antiglobulin test, the workup of significant neonatal hyperbilirubinemia involves specialized tests with delayed results or results that can be inconclusive in the neonatal period. Hereditary spherocytosis (HS), a heterogeneous disorder caused by various erythrocyte membrane protein defects resulting in membrane instability and the formation of spherocytes,1  frequently presents with neonatal hyperbilirubinemia2  but is often not diagnosed until long after the neonatal period.3,4 

The diagnosis of HS in neonates can be difficult because of the absence of anemia at birth,2,5  poor sensitivity and specificity of the presence of spherocytes on a peripheral blood smear,1,6,7  variable reticulocytosis,1,5  and unreliable osmotic fragility testing in newborns, with false-positives occurring in immune hemolytic anemia and red blood cell (RBC) enzyme deficiencies.8,9  Early suspicion and expeditious diagnosis of HS could allow for parental education and greater physician awareness, which may reduce the risk of kernicterus because of earlier initiation of aggressive phototherapy and may lead to prompt detection of aplastic crises and avoidance of hospitalization for transfusion.10 

The Neonatal HS Index (hereafter, “HS Index”), described by Christensen et al,6,11  can be obtained from a complete blood count (CBC) (recommended by the American Academy of Pediatrics in the evaluation of neonates requiring phototherapy12 ) simply by dividing the mean corpuscular hemoglobin concentration (MCHC) by the mean corpuscular volume (MCV). In a recent review6  of HS, researchers suggested using the HS Index in the evaluation of neonates with problematic jaundice, defined as jaundice requiring phototherapy for >2 days.6  On the basis of the report that an HS Index >0.36 had 97% sensitivity and >99% specificity for identifying HS11  (among a population of mostly white neonates13 ), further diagnostic testing for HS was recommended among such neonates if the HS Index was >0.36.6  Previous studies have revealed that neonates and children >12 months with HS have an abnormally high MCHC as a result of mild cellular dehydration.7,14  Christensen et al11  reported lower MCV values among neonates with HS, whereas, in previous studies, researchers have found the MCV in children to be nondiscriminative for HS.9,15 

We sought to reevaluate the utility of the HS Index for several reasons. First, the validity of the study reporting high sensitivity and specificity of the HS Index may have been compromised by the inclusion of all CBCs (n = 182) drawn on infants with HS (n = 33) over the course of 90 days but only the initial CBC drawn on the non-HS infants.11  It is likely that the initial CBC obtained on non-HS infants was drawn during the first few days after birth, while infants were being assessed for sepsis or jaundice. Because the MCV decreases from ∼119 fL at birth to 88 fL over the first 90 days,16  CBCs sent on HS infants at later ages could result in their HS Index being higher simply because they were older and, hence, had a lower MCV. Second, information is lost when dichotomizing the HS Index (ie, identifying a particular threshold as abnormal) because the difference between equivocal and extremely abnormal results is not quantified. Third, the usefulness of the HS Index may be affected by hyperbilirubinemia. Because the HS Index will largely be used in infants with significant hyperbilirubinemia, its test characteristics in that group are particularly relevant. Finally, the Kaiser Permanente Northern California (KPNC) population provides the opportunity to study the HS Index in an ethnically diverse population. The objective of this study was to evaluate the utility of the HS Index obtained from the initial CBC within 7 days after birth in a diverse population by using interval likelihood ratios (LRs)17  for various HS Index ranges, stratifying by hyperbilirubinemia.

The study was approved by the institutional review boards at KPNC (1270417-17) and the University of California, San Francisco (16-19246). We obtained data for this retrospective cohort study from KPNC medical records and demographic, laboratory, and hospitalization databases.

We included all infants born at ≥35 weeks’ gestation from 1995 through 2015 at 15 KPNC hospitals. KPNC is an integrated community medical care delivery system that serves 4.3 million members, nearly one-half of the insured population of Northern California. Neonates were eligible for the primary analysis if they had a CBC with erythrocyte indices drawn within the first 7 days after birth.

CBCs at KPNC hospitals are performed by using Beckman Coulter (Brea, CA) or Sysmex Corporation (Munderlein, IL) hematology analyzers. We included only the first CBC with erythrocyte indices drawn per subject (within the first 7 days) to avoid the possible confounding effects of transfused erythrocytes (because we expected most transfusions would occur after an initial CBC) and declining MCV values over time. We also collected MCV data from CBCs drawn within the first 30 days after birth to determine how the MCV changes over time. The HS Index was calculated by dividing the MCHC (expressed as grams hemoglobin per deciliter RBC) by the MCV (expressed as femtoliters). We obtained the highest total serum bilirubin (TSB) value drawn during the first month after birth for each subject. We excluded any TSB measurements for which corresponding conjugated or direct bilirubin measurement constituted ≥50% of the TSB level because these levels suggest liver disease as a cause of hyperbilirubinemia. Universal bilirubin screening began in some facilities in September 2004 and was fully implemented in all facilities by February 2007.18  Additional laboratory and clinical characteristics obtained from electronic records included direct antiglobulin test results, sex, gestational age, birth weight, maternal and infant race and ethnicity, receipt of phototherapy, readmission for phototherapy, and receipt of an exchange transfusion or RBC transfusion. Infant race and/or ethnicity was used as a proxy variable to reflect the varying risk of HS on the basis of ancestry.

The outcome variable assessed was the presence or absence of HS. We identified patients with possible HS by searching KPNC electronic records for (1) encounters with International Classification of Diseases, Ninth Revision or International Classification of Diseases, 10th Revision codes for HS (282.0 or D58.0, respectively) and (2) subjects without HS International Classification of Diseases, Ninth Revision or International Classification of Diseases, 10th Revision codes but who had osmotic fragility and/or ektacytometry tests ordered (the 2 most common HS confirmatory tests performed at KPNC) during follow-up. We then verified that these patients have HS by reviewing their electronic and paper charts, with particular attention to pediatric hematology/oncology consult notes. We collected data on the age at diagnosis, timing of RBC transfusion(s), hospital admission for RBC transfusion, hemoglobin values, and splenectomy. We required either (1) a positive diagnostic test (osmotic fragility, ektacytometry, eosin-5'-maleimide [EMA] binding, and/or DNA sequencing) or (2) a positive family history and typical clinical features or laboratory findings (receipt of an RBC transfusion, splenectomy, anemia, elevated reticulocytes, and/or elevated spherocytes on serial CBCs) to confirm a diagnosis of HS. Deidentified study data were collected and managed by using the Research Electronic Data Capture (REDCap) tools hosted at the University of California, San Francisco.

We compared demographic, laboratory, and clinical characteristics of infants with and without HS by using t tests or χ2 tests as appropriate. We estimated the prevalence of HS by race and ethnicity to determine the previous probability of HS for each race and ethnicity and compared these previous probabilities via χ2 tests. We divided our cohort into 4 separate groups on the basis of highest TSB value (TSB values: <10 mg/dL, 10–14.9 mg/dL, 15–19.9 mg/dL, and ≥20 mg/dL). We performed χ2 tests to determine if there is a difference in the previous probability of HS between TSB groups. We expected the prevalence of HS to increase with rising peak TSB values.

We quantified the discrimination of the HS Index by using the area under the receiver operating characteristic curve (AUROC). We estimated LRs for different HS Index intervals irrespective of peak TSB level. HS Index intervals were chosen on the basis of inflection points in the observed receiver operating characteristic curve. LRs were estimated as the proportion of HS cases divided by the proportion of noncases for a particular HS Index interval.17  We estimated LRs for the various peak TSB categories irrespective of HS Index values so that we could take into account severity of hyperbilirubinemia when determining the probability of HS. We compared the discrimination of the HS Index by peak TSB levels by comparing the AUROC. Although not specified a priori, we evaluated the discrimination of the red cell distribution width (RDW) and estimated LRs for various RDW intervals. We performed all analyses by using Stata/SE 14 (Stata Corp, College Station, TX), supplemented by a Stata command that we developed to calculate interval LRs (available from the authors).

Of the 670 272 infants included in the study on the basis of their year of birth and gestational age, 79 infants were diagnosed with HS (Fig 1), resulting in a prevalence of 1.2 per 10 000 infants. There were 5 infants whose diagnosis was unclear because of loss to follow-up; these infants were excluded from the analysis. Of the subjects with HS, 50 infants had a CBC with erythrocyte indices performed within 7 days of birth, 63 infants had a TSB within 30 days of birth, and 48 infants had both a CBC and TSB. Characteristics of the newborns with and without HS included in the study are shown in Table 1. The prevalence of HS was higher among infants of white and “other” (presumably multiple) race and ethnicity categories and increased with higher peak serum bilirubin levels (P < .001 for linear trend; Table 1). The risk of HS was higher among infants needing phototherapy, requiring readmission for phototherapy, with TSB values that crossed the exchange transfusion threshold and who received an exchange transfusion or an RBC transfusion during the first month of life.

FIGURE 1

Selection of infants with HS. ICD-9, International Classification of Diseases, Ninth Revision; ICD-10, International Classification of Diseases, 10th Revision.

FIGURE 1

Selection of infants with HS. ICD-9, International Classification of Diseases, Ninth Revision; ICD-10, International Classification of Diseases, 10th Revision.

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TABLE 1

Study Demographics and Predictors of HS

HSNo HSHS Risk per 10 000Risk Ratio95% CIP
Entire study, n 79 670 188 1.18 — — — 
Male, n 40 342 519 1.17 1.0 0.6–1.5 .93 
Gestational age, mean (SD), wk 39.0 (1.4) 39.1 (1.4) — — — .49 
Birth wt, mean (SD), kg 3.36 (0.55) 3.43 (0.51) — — — .25 
Race and ethnicity, n      .01 
 White 46 275 638 1.67 1.0 — — 
 Asian 130 217 0.46 0.3 0.1–0.6 .002 
 African American 50 456 0.99 0.6 0.2–1.5 .26 
 Hispanic 14 161 162 0.87 0.5 0.3–0.9 .03 
 American Indian or Pacific Islander 2203 — .54 
 Other 33 490 2.09 1.3 0.6–2.8 .58 
 Unknown 17 022 0.59 0.4 0.05–2.6 .28 
CBC (with HS Index) sent at ≤7 d, n 50 128 770 3.88 7.2 4.6–11.5 <.001 
Age at first CBC sent at ≤7 d, mean (SD), hr 31.9 (26.2) 17.4 (28.9) — — — <.001 
No. CBCs within 30 d, mean (SD) 2.9 (4.1) 0.3 (0.9) — — — <.001 
TSB sent at ≤30 d, n 63 401141 1.57 2.6 1.5–4.6 <.001 
Maximum TSB in mg/dL, n      <.001 
 no bilirubin 16 269 047 0.59 1.0 — — 
 <10 189 295 0.21 0.4 0.1–1.1 .05 
 ≥10 to <15 20 117 086 1.71 2.9 1.5–5.5 .001 
 ≥15 to <20 23 83 501 2.75 4.6 2.4–8.8 <.001 
 ≥20 16 11 240 14.21 23.9 12.0–47.8 <.001 
No. TSBs per patient, n, mean (SD) 7.0 (6.6) 1.5 (2.0) — — — <.001 
Any phototherapy, n 47 55 130 8.52 16.4 10.4–25.7 <.001 
Readmission for phototherapy, n 11 14 729 7.46 7.2 3.8–13.6 <.001 
Crossed exchange transfusion threshold, n 1986 15.08 10.0 3.1–32.0 <.001 
Exchange transfusion, n 145 68.49 58.7 8.2–419 <.001 
RBC transfusion during first 30 d, n 1062 74.77 70.5 34.0–146 <.001 
HSNo HSHS Risk per 10 000Risk Ratio95% CIP
Entire study, n 79 670 188 1.18 — — — 
Male, n 40 342 519 1.17 1.0 0.6–1.5 .93 
Gestational age, mean (SD), wk 39.0 (1.4) 39.1 (1.4) — — — .49 
Birth wt, mean (SD), kg 3.36 (0.55) 3.43 (0.51) — — — .25 
Race and ethnicity, n      .01 
 White 46 275 638 1.67 1.0 — — 
 Asian 130 217 0.46 0.3 0.1–0.6 .002 
 African American 50 456 0.99 0.6 0.2–1.5 .26 
 Hispanic 14 161 162 0.87 0.5 0.3–0.9 .03 
 American Indian or Pacific Islander 2203 — .54 
 Other 33 490 2.09 1.3 0.6–2.8 .58 
 Unknown 17 022 0.59 0.4 0.05–2.6 .28 
CBC (with HS Index) sent at ≤7 d, n 50 128 770 3.88 7.2 4.6–11.5 <.001 
Age at first CBC sent at ≤7 d, mean (SD), hr 31.9 (26.2) 17.4 (28.9) — — — <.001 
No. CBCs within 30 d, mean (SD) 2.9 (4.1) 0.3 (0.9) — — — <.001 
TSB sent at ≤30 d, n 63 401141 1.57 2.6 1.5–4.6 <.001 
Maximum TSB in mg/dL, n      <.001 
 no bilirubin 16 269 047 0.59 1.0 — — 
 <10 189 295 0.21 0.4 0.1–1.1 .05 
 ≥10 to <15 20 117 086 1.71 2.9 1.5–5.5 .001 
 ≥15 to <20 23 83 501 2.75 4.6 2.4–8.8 <.001 
 ≥20 16 11 240 14.21 23.9 12.0–47.8 <.001 
No. TSBs per patient, n, mean (SD) 7.0 (6.6) 1.5 (2.0) — — — <.001 
Any phototherapy, n 47 55 130 8.52 16.4 10.4–25.7 <.001 
Readmission for phototherapy, n 11 14 729 7.46 7.2 3.8–13.6 <.001 
Crossed exchange transfusion threshold, n 1986 15.08 10.0 3.1–32.0 <.001 
Exchange transfusion, n 145 68.49 58.7 8.2–419 <.001 
RBC transfusion during first 30 d, n 1062 74.77 70.5 34.0–146 <.001 

P values are calculated by χ2 tests for categorical variables and t tests for continuous variables. —, not applicable.

The majority of infants diagnosed with HS (85%, n = 67) had confirmatory testing performed. A diagnosis was made because of a positive family history of HS and typical clinical or laboratory findings in 15% (n = 12) of HS subjects (Supplemental Table 5). Among subjects with HS, 78% had a positive family history, 80% were diagnosed by 5 years of age, 59% received phototherapy, 29% received an RBC transfusion, and 30% underwent splenectomy during follow-up (Table 2). The median follow-up time for the HS subjects was 7.2 years (interquartile range: 3.1–14.4). The majority of RBC transfusions occurred during the first year after birth and led to admission to the hospital (Table 2).

TABLE 2

Clinical Characteristics of HS Subjects (n = 79)

n%
Family history 62 78 
Age at diagnosis   
 <1 y 36 46 
 1–5 y 27 34 
 6–10 y 
 11–15 y 
 ≥16 y 
 Unclear 11 
Phototherapy 47 59 
RBC transfusion during follow-up 23 29 
RBC transfusion previous to 1 y 15 19 
Admitted to hospital for RBC transfusion 20 25 
Hb <7 mg/dL on any CBC during follow-up 27 34 
Splenectomy 24 30 
Median follow-up time, y (IQR) 7.2 3.1–14.4 
n%
Family history 62 78 
Age at diagnosis   
 <1 y 36 46 
 1–5 y 27 34 
 6–10 y 
 11–15 y 
 ≥16 y 
 Unclear 11 
Phototherapy 47 59 
RBC transfusion during follow-up 23 29 
RBC transfusion previous to 1 y 15 19 
Admitted to hospital for RBC transfusion 20 25 
Hb <7 mg/dL on any CBC during follow-up 27 34 
Splenectomy 24 30 
Median follow-up time, y (IQR) 7.2 3.1–14.4 

Hb, hemoglobin; IQR, interquartile range.

There were significant differences in hemoglobin, hematocrit, RBC indices, HS Index values, and RDWs between neonates with and without HS (Table 3). The mean HS Index value of neonates with HS was higher than that of the reference group at 0.358 (SD: 0.023) vs 0.327 (SD: 0.029). However, there is a substantial amount of overlap when comparing HS Index distribution by HS status (Fig 2). HS infants had a mean MCV of 99.1 fL vs 104.8 fL among non-HS infants. If we included all CBCs (n = 163) drawn per HS subject within the first 30 days of birth, the mean MCV decreased to 95.3 fL. Discrimination of the HS Index as a continuous variable was good, with an AUROC of 0.84 (95% confidence interval [CI] 0.78–0.90; Fig 3). The LRs for the HS Index ranged from 10.1 for an HS Index ≥0.380 to 0.1 for an HS Index <0.310 (Table 4). Dichotomized at 0.36, the HS Index had sensitivity of 56%, specificity of 93%, and positive predictive value of 0.3%.

FIGURE 2

HS Index distribution among CBCs sent within 7 days of birth.

FIGURE 2

HS Index distribution among CBCs sent within 7 days of birth.

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FIGURE 3

Receiver operating characteristic curves for RDW, HS Index, and peak TSB. RDW and HS Index values obtained from the first CBC drawn within 7 days after birth. Peak TSB values obtained from the highest TSB value drawn during the first month after birth. CI, confidence interval.

FIGURE 3

Receiver operating characteristic curves for RDW, HS Index, and peak TSB. RDW and HS Index values obtained from the first CBC drawn within 7 days after birth. Peak TSB values obtained from the highest TSB value drawn during the first month after birth. CI, confidence interval.

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TABLE 3

RBC Indices (From First CBC Within 7 d of Birth)

HS (n = 50), Mean (SD)No HS (n = 128 770), Mean (SD)PAUROC
Hct, % 46.6 (6.5) 51.1 (7.0) <.001 0.68 
MCV, fL 99.1 (5.2) 104.8 (5.3) <.001 0.80 
MCHC, g/dL 35.4 (1.1) 34.2 (1.2) <.001 0.80 
HS Index, MCHC ÷ MCV 0.358 (0.023) 0.327 (0.029) <.001 0.84 
RDW,a20.1 (2.0) 16.9 (1.6) <.001 0.91 
HS (n = 50), Mean (SD)No HS (n = 128 770), Mean (SD)PAUROC
Hct, % 46.6 (6.5) 51.1 (7.0) <.001 0.68 
MCV, fL 99.1 (5.2) 104.8 (5.3) <.001 0.80 
MCHC, g/dL 35.4 (1.1) 34.2 (1.2) <.001 0.80 
HS Index, MCHC ÷ MCV 0.358 (0.023) 0.327 (0.029) <.001 0.84 
RDW,a20.1 (2.0) 16.9 (1.6) <.001 0.91 

P values are calculated by using the Wilcoxon rank test. Hct, hematocrit.

a

n = 43 for HS subjects; n = 104 852 for non-HS subjects.

TABLE 4

Interval LR for HS Index, Peak TSB, and RDW

No. With HSNo. Without HSLR95% CI
HS Index Interval     
 ≥0.380 2040 10.1 5.3–19.1 
 ≥0.360 to <0.380 20 6587 7.8 5.6–1.0 
 ≥0.355 to <0.360 3493 3.7 1.6–8.5 
 ≥0.335 to <0.355 28 683 0.7 0.4–1.4 
 ≥0.310 to <0.335 62 939 0.3 0.2–0.6 
 <0.310 25 073 0.1 0.02–0.7 
Peak TSB interval (mg/dL)     
 ≥20 16 11 240 9.1 5.9–13.8 
 ≥15 to <20 23 83 501 1.8 1.3–2.4 
 ≥10 to <15 20 117 086 1.1 0.8–1.6 
 <10 189 295 0.1 0.05–0.3 
RDW interval (%)     
 ≥20 24 3875 15.1 11.6–19.7 
 ≥19 to <20 4830 4.0 2.2–7.6 
 ≥18 to <19 11 564 1.5 0.7–2.9 
 ≥17 to <18 24 238 0.2 0.05–0.8 
 <17 60 345 0.08 0.02–0.3 
No. With HSNo. Without HSLR95% CI
HS Index Interval     
 ≥0.380 2040 10.1 5.3–19.1 
 ≥0.360 to <0.380 20 6587 7.8 5.6–1.0 
 ≥0.355 to <0.360 3493 3.7 1.6–8.5 
 ≥0.335 to <0.355 28 683 0.7 0.4–1.4 
 ≥0.310 to <0.335 62 939 0.3 0.2–0.6 
 <0.310 25 073 0.1 0.02–0.7 
Peak TSB interval (mg/dL)     
 ≥20 16 11 240 9.1 5.9–13.8 
 ≥15 to <20 23 83 501 1.8 1.3–2.4 
 ≥10 to <15 20 117 086 1.1 0.8–1.6 
 <10 189 295 0.1 0.05–0.3 
RDW interval (%)     
 ≥20 24 3875 15.1 11.6–19.7 
 ≥19 to <20 4830 4.0 2.2–7.6 
 ≥18 to <19 11 564 1.5 0.7–2.9 
 ≥17 to <18 24 238 0.2 0.05–0.8 
 <17 60 345 0.08 0.02–0.3 

Discrimination of the HS Index was not significantly different among infants who did and did not receive phototherapy (P = .9). The HS Index appeared to perform best among infants with TSB levels <10 mg/dL and was similar among the other categories (P < .001). LRs for the peak TSB categories are shown in Table 4. When we dichotomized the HS Index (at ≥0.355) and the TSB levels (at ≥15 mg/dL), the LRs for all 4 combinations of results were similar to those predicted from the products of the LRs for each test result, suggesting that the HS Index and TSB are independent tests for HS (Supplemental Table 6).

Because the difference in mean RDW between neonates with and without HS was substantial (20.0% vs 16.9%), we also evaluated the discrimination of RDW, which was excellent at 0.91 (95% CI 0.86–0.96) and better than that of the HS Index (Fig 3). LRs for the RDW categories are shown in Table 4.

In this retrospective cohort study, we found that the sensitivity and specificity of the HS Index were lower than previously reported and its usefulness limited by the low previous probability of HS. Consider a white infant with no family history of HS who has a peak serum bilirubin of 20 mg/dL and an HS Index of 0.38. Assuming the peak TSB and HS are independent tests for HS, we can multiply the pretest odds of HS among white infants in our population (1.67 per 10 000) by the LR for TSB ≥20 mg/dL (9.1) and LR for HS Index ≥0.380 (10.1) to obtain the post-test probability. Even with both tests abnormal, the probability of HS in this infant remains low at ∼1.5% (0.000167 × 9.1 × 10.1) and unlikely to prompt us to pursue further diagnostic testing.

On the other hand, obtaining an HS Index or RDW costs little and, if elevated, may prompt a physician to treat jaundice more aggressively. For example, one might treat an infant with a high HS Index and/or high RDW, high bilirubin, and elevated reticulocytes as having hemolytic disease (particularly if there is a family history of HS), thus starting phototherapy at a lower bilirubin level.

The HS Index may be more useful in ruling out HS in neonates who have a family history of HS. If, for example, an infant has both a low HS Index of <0.310 and a TSB <10 mg/dL, the probability of HS would decrease from 50% (assuming autosomal dominant HS) to ∼1% (previous odds of HS × LR for HS Index <0.310 × LR for TSB <10 = 1 × 0.1 × 0.1). Therefore, we could be reassured that the infant is unlikely to have inherited HS and would defer confirmatory studies.

There are several possible explanations for the lower utility of the HS Index in our study compared with that of the report by Christensen et al.6,11  Our population had a lower prevalence of HS, at 1.2 per 10 000. This is partially explained by the diversity of KPNC members, with a lower proportion of white members (41%), who have the highest incidence of HS. However, even among white infants, the prevalence of HS at ∼1.7 in 10 000 was lower than the expected 2 to 5 in 10 000. It is possible that we missed diagnoses among subjects with mild, asymptomatic HS. Furthermore, infants born at the later end of the cohort may not have undergone workup for HS, either because they had not yet become clinically symptomatic or because diagnostic testing is usually performed after 12 months of age. Newer DNA sequencing panels were rarely ordered in our cohort, which may have resulted in missed cases because of equivocal or negative results on other confirmatory tests or misdiagnosis as another type of hereditary anemia.9,19  We also did not investigate whether subjects with substantially elevated HS Index and/or RDW values had clinical findings consistent with an undiagnosed case of HS. However, even in the unlikely event that we missed as many as 20% of HS cases, the main effect on our results would be only a slight (∼20%) underestimation in the previous and posterior probability of HS, which would not affect our conclusions.

Our decision to include only the first CBC drawn per infant in the first week after birth had a large impact on our disparate findings. Because neonatal hyperbilirubinemia peaks during the first week, CBCs to evaluate jaundice are likely to be drawn then as well, making them the most useful in screening for HS. In Christensen’s cohort, the mean MCV among HS infants was 89.9 fL (compared with 107 fL among the reference group),11  whereas, in our cohort, HS infants had a mean MCV of 99.1 fL (compared with 104.8 fL among non-HS infants). As expected, the mean MCV among HS infants in our cohort fell to 95.4 fL when we included all CBCs drawn per HS subject within the first 30 days and would likely decrease further had we included CBCs out to 90 days. This supports our hypothesis that the inclusion of multiple CBCs for HS infants and only the initial CBC for the reference group contributed to the difference in HS Index values between the 2 groups.

In previous studies, it has been revealed that an elevated RDW (reflective of anisocytosis) is typical among individuals with HS.8  In children >1 year of age, an RDW >14% had an 85% sensitivity and 97% specificity for identifying HS, with a LR of 28.3.14  However, the utility of the RDW among neonates with HS has not, to our knowledge, been previously described. Among 13 neonates in the Intermountain Healthcare System with HS who had a TSB in the high-risk zone20  and a CBC drawn within the first week of life, the mean RDW was elevated at 20.7% (SD: 1.5%),7  but the authors reported RDW had poor discrimination for identifying HS. In our cohort, the mean RDW among HS infants was similar at 20.1% (SD: 2.0%) but had better discrimination than the HS Index. It is important to note that RBC indices, in particular, the MCV (which is used to calculate the hematocrit and MCHC), vary slightly on the basis of the make and model of hematology analyzer used.2123  Differences between instruments may affect the precision of the intervals used to calculate LRs for the HS Index and RDW.

The discrimination of the HS Index was modest in neonates when obtained from a CBC drawn within the first week after birth. Although the RDW performed better than the HS Index as a screening tool in neonates, its usefulness is also limited by the low previous probability of HS, which is only modestly raised by hyperbilirubinemia. However, because both of these screening tests have minimal cost, they may be helpful in situations in which even a low post-test probability would alter management, such as prompting more aggressive treatment of jaundice, while performing further workup.

We thank Hamid Niki for constructing the study’s background data sets from electronic data.

Dr Weiss assisted with study design, designed the data collection instrument, performed chart review, conducted the initial analyses, drafted the initial manuscript, and reviewed and revised the manuscript; Dr Kuzniewicz contributed to the acquisition of data, provided statistical consultation, and reviewed and revised the manuscript; Dr Shimano contributed to study design, including design of the data collection instrument, and reviewed and revised the manuscript; Ms Walsh coordinated data retrieval, assisted with chart review, and reviewed and revised the manuscript; Dr Newman conceptualized and designed the study, assisted with statistical analyses, and reviewed and revised the manuscript; and all authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.

FUNDING: No external funding.

COMPANION PAPER: A companion to this article can be found online at www.pediatrics.org/cgi/doi/10.1542/peds.2021-051100.

     
  • AUROC

    area under the receiver operating characteristic curve

  •  
  • CBC

    complete blood count

  •  
  • EMA

    eosin-5'-maleimide

  •  
  • HS

    hereditary spherocytosis

  •  
  • KPNC

    Kaiser Permanente Northern California

  •  
  • LR

    likelihood ratio

  •  
  • MCHC

    mean corpuscular hemoglobin concentration

  •  
  • MCV

    mean corpuscular volume

  •  
  • RDW

    red cell distribution width

  •  
  • TSB

    total serum bilirubin

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Competing Interests

POTENTIAL CONFLICTS OF INTEREST: The authors have indicated they have no potential conflicts of interest to disclose.

FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.

Supplementary data