A retrospective comparison of phototherapy need in O–B versus O–A incompatibility in a single Saudi institution

1 Introduction

ABO incompatibility is a major risk factor for neonatal indirect hyperbilirubinemia (NIH), requiring treatment [1–6]. ABO incompatibility occurs when anti-A or anti-B immunoglobulin G (IgG) of mothers’ O blood group sensitizes newborns’ erythrocytes blood group A (O–A incompatibility) or B (O–B incompatibility), respectively [4, 5].

It has been shown that there are racial differences between O–B and O–A neonatal incompatibility. The O–B incompatibility is associated with a higher rate of direct antiglobulin test (DAT) positive in the black population [7]. In contrarily, the O–A incompatibility is associated with a higher rate of DAT positive in Italians and Greece [8, 9]. In Turkish, the rate of DAT positive was the same in both O–B and O–A incompatibility [10]. Some studies have found that O–B incompatibility is associated with a higher rate of phototherapy need than O–A incompatibility [4, 5, 11], but other studies have not found this association [7–10, 12–15].

A recent Saudi retrospective study from a neighboring city (Al-Khobar) compared the O–B with the O–A incompatibility [16]. This neighbors’ study showed that the O–B had associated with more positive DAT and phototherapy needs than the O–A incompatibility. It demonstrated that phototherapy requirement was the same among DAT positive and negative O–B incompatibility contradicting the well-established association between DAT positivity and phototherapy requirement [6]. This unexpected finding could be due to three limitations which the first two of them were acknowledged by the authors. The first limitation was that glucose-6-phosphate dehydrogenase (G6PD) deficiency contaminated the study sample as the G6PD deficiency is very common in the eastern province and the included neonates were not screened for G6PD deficiency. The second limitation was the small number of O–B (n = 86) and the O–A (n = 111) incompatibility included in the study. The third limitation was that the study included 36 and 37 weeks of gestation, and these gestational ages are independently associated with greater odds for phototherapy requirements than≥38 weeks of gestation [6]. Therefore, we aimed to compare DAT positivity and phototherapy need in O–B with versus (vs) incompatibility in a larger study from the Eastern Province in Saudi Arabia that included neonates born at≥38 weeks of gestation with normal G6PD levels.

2 Methods

3 Results

A total of 611 neonates met our inclusion and exclusion criteria. The proportion of male and female neonates was the same [Males = 319 (52.2%), Females = 292 (48.8%), binomial p-value = 0.29]. The portion of O–A and O–B incompatibility were the same [303 (49.6%) vs 308 (50.4%), binomial p-value = 0.87]. Table 1 depicts the baseline characteristics and laboratory findings of the O–A and O–B incompatibility.

Of the 222 (36.3%) neonates that had positive DAT, 160 (72%) was DAT– 1 + and 62 (28%) was DAT– 2 + . Positive DAT was more prevalent in the O–B than the O–A incompatibility [43.5% (134/308) vs 29.2% (88/301), OR = 1.864, 95% CI: 1.333– 2.607, p < 0.001). Frequency of DAT positive was the same among males and females in the O–B 39.8% (68/171) vs 48.2% (66/137), OR = 1.41, 95% CI: 0.89– 2.22, 0.14] and the O–A incompatibility [24.7% (36/146) vs 33.5% (52/155), OR = 1.54, 95% CI: 0.933– 2.55, 0.09].

A total of 109 (17.8%) neonates were treated with phototherapy during their first 14 days of life. The rate of phototherapy need was the same in both male and female neonates (17.2% vs 17.8%, p = 0.92). A greater odd of phototherapy need was observed in the O–B vs O–A incompatibility across various strata except for DAT negative, where the phototherapy need was the same in both incompatibilities (Table 2). The increased phototherapy needs in the DAT positive O–B vs DAT positive O–A (46.3% vs 19.3%) was disproportionate to that in their DAT negative counterparts (8.6% vs 7.0%). Readmission for NIH, use of 360° exposure cylindrical intensive phototherapy machine, or intravenous immunoglobulin (IVIG) administration was more prevalent in the O–B than the O–A incompatibility (13.2% vs 5.0%, OR = 2.90, 95% CI: 1.56– 5.39, p < 0.001).

Table 3 depicted the results of hierarchical binary logistic regression analysis for phototherapy need as a dependent variable. There was no important difference between the estimated coefficient of DAT positive in model-1 (1.883) and model-2 (1.801) that included ABO incompatibility. The result of model-3 showed that the O–B incompatibility×DAT positive interaction was statistically significant. These observations indicate that the ABO incompatibility status was a mediator effect on the relationship between DAT positivity and the need for phototherapy. Accordingly, OR of DAT positive for the phototherapy need = Exponential (coefficient of the DAT plus coefficient of the interaction) = Exp (1.151 + 1.061) = 9.13 with a 95% CI of 4.80–17.39.

We presented the evidence that the O–B incompatibility had a mediator effect on the relationship between DAT positivity and the need for phototherapy in the study population. The DAT positive O–B had greater odds of phototherapy need than DAT positive O-–A incompatibility. These findings emphasize that the O–B and O–A are not the same from the NIH point of view.

An important step in the process of modeling a set of data is determining whether there is evidence of confounding and/or interaction in the data [21]. Table 2 showed that the ORs of DAT positive for photo therapy needs were not constant in O–A and O–B incompatibility strata.

This suggests that there may be either confounding or effect modification due to ABO incompatibility, or both. Biologically, the ABO incompatibility is not a confounder because it is part of the causal pathway between DAT positive and the phototherapy needs [22]. Statistically, Table 3 showed that the ABO incompatibility was not a confounder as the estimated coefficient of DAT positive was nearly the same after including the ABO incompatibility in model-2. The model-3 suggests that the ABO incompatibility was an effect modifier as the O–B incompatibility×DAT positive interaction was statistically significant. In other words, the estimate of the OR for DAT positive depends on the type of ABO incompatibility. Thus, we may not be able to estimate the OR by simply exponentiating an estimated coefficient of DAT. Alternatively, the correct OR can be estimated by exponentiating the sum of the coefficient of the DAT and the coefficient of the interaction as we demonstrated in the result section.

Indeed, it is biologically plausible that the O–B incompatibility is mediator effect and associated with more DAT positivity and phototherapy need than the O–A incompatibility. The blood group A and B antigens are expressed on the platelets matching the individual’s erythrocytes ABO blood group [23–28]. Most individuals are lower expressers of A and B antigens on platelets; however, about 30% are moderate expressors and 7% are high expressors [23, 24, 28]. The mothers’ anti-A or anti-B IgG sensitize the platelets similar to erythrocytes which may cause severe neonatal thrombocytopenia [29–32].

Other researchers have proposed that the mothers’ anti-A or anti-B IgG is diluted by sensitizing the platelets and other cells that express the A and B antigens [4, 33, 34]. It has been shown that the B antigen is less expressed on platelets and has lower antigenicity than the A antigens [24, 28, 35]. Assuming the titer of the mother’s IgG is steady, dilution of mothers’ anti-B is expected to be less than the mothers’ anti-A; thus, more IgG sensitizing neonatal erythrocytes yield more DAT positivity and more phototherapy needed in O–B than O–A incompatibility. The mothers’ anti-A and anti-B IgG were the culprits for severe neonatal thrombocytopenia in four cases [29, 31, 32]. Of these cases, the NIH was specified in three [31, 32]. Interestingly, although the anti-A and anti-B IgG titer were very high, the phototherapy treatment was required in one case only [32]. All these suggest that level of expression of A and B antigens on platelets may play a role in NIH related to ABO incompatibility in addition to the imbalance between bilirubin production (hemolysis) and excretion.

It has been observed that ABO incompatibility was unlikely to be the cause of severe NIH that required phototherapy or caused acute kernicterus if DAT was negative [33, 36]. This observation is supported by our finding of a disproportionate increase in phototherapy needs in the O–B vs O–A incompatibility across the DAT strata. If ABO incompatibility was the cause of phototherapy need in DAT negative cases, then it is expected that the magnitude of increase in phototherapy need would be proportionate in O–B vs O–A across DAT strata.

Our study and the neighbors’ study found that DAT positivity was significantly more prevalent among the O–B than the O–A incompatibility [16]. We observed that the O–B incompatibility associated with the phototherapy needs except when DAT was negative whereas the neighbors’ study found that the O–B incompatibility was associated with phototherapy needs regardless of DAT results [16]. This discrepancy could be due to its the three shortcomings that were outlined in the introduction and overcome in our study.

In addition to being a retrospective study, other limitations of the present study should be noted. First, no data was available on other possible effect modifiers or confounders for phototherapy needs, such as type of feeding. Second, cord DAT positivity was not confirmed by a neonatal venous sample to exclude the little possibility small false positive due to Wharton’s jelly contamination [37]. Third, DAT negativity was not confirmed by elute technique [38, 39]. Fourth, we did not collect data on the duration of phototherapy treatment. Fifth, we did not adjust p values for our multiple comparisons, which might increase the risk of false statistical significance [40]. However, many of our p values are so small that they would still be statistically significant even if the Bonferroni method, the most conservative, had been used.

In conclusion, the present study supports that the O–B incompatibility had a mediator effect on the relationship between DAT positivity and the need for phototherapy in the study population. The DAT positive O–B had greater odds of phototherapy need than DAT positive O-–A incompatibility. These findings emphasize that the O–B and O–A are not the same from the NIH point of view.