Neonatal jaundice is the most common clinical condition detected in neonatology, as it is present in more than 60 % of neonates. Even though it commonly results from physiologic changes after the birth, the identification of other possible pathologic reasons for icterus is very important. In case of haemolysis, which is a common pathologic factor for neonatal jaundice, high levels of bilirubin and anaemia can be expected. High level of unbound bilirubin can be potentially dangerous for the neonate, since it can cause bilirubin-induced neuronal damage. Haemolysis augments the risk of bilirubin neurotoxicity, but the mechanism of this intensifying effect is uncertain. Consequently, haemolysis represents an important risk factor for development of acute and chronic bilirubin encephalopathy. Confirmation of haemolysis in neonate is therefore very important in terms of explaining the aetiology of the jaundice as well as for its management.
Confirming haemolysis in neonates can be very challenging. Introduction of a reliable and simple diagnostic test for the confirmation of haemolysis would therefore be very appreciated in the management of neonatal jaundice. During haeme degradation process carbon monoxide (CO) is produced and bound to haemoglobin, forming carboxyhaemoglobin (COHb). The COHb level in blood can serve as an index of endogenous produced CO and haemolysis. Determination of COHb with CO-oximetry is fast, simple and widely available laboratory method, needing small amount of blood. Determination of COHb with CO-oximeters in neonates hasn’t been adequately studied. Defining the role of COHb measured with CO-oximetry in neonates in evaluation of haemolysis is therefore an interesting research challenge to which the present study was aimed.
In the introduction, pathophysiology of physiologic icterus, the commonest reasons for the pathologic icterus and the aetiology and mechanisms of haemolytic process in haemolytic disease of the newborn are presented. Furthermore, the mechanisms of bilirubin induced neurologic damage are discussed and studies confirming the role of haemolysis in the development of bilirubin encephalopathy are presented. In neonates, haemolysis is detected with indirect haematological and specific transfusion laboratory tests. Clinical dilemmas when using these tests in neonates are discussed. Haeme degradation process and the results of clinical studies demonstrating increased formation of CO during haemolysis detected with elevated COHb levels in blood or elevated CO in exhaled breath air are presented. In clinical laboratories COHb is determined spectrophotometrically with CO-oximetry, while for research purposes COHb has been more often determined with gas chromatography. Degradation of haeme is catalysed by haeme oxygenase (HO). The HO-1 isoform is an inducible enzyme; clinical factors potentiality influencing the enzyme transcription and consequently influencing the CO formation and COHb levels in the body are discussed.
In the second part, methods and results of the study, consisting of two parts, are presented. In the prospective part of the study, the COHb values measured with CO-oximetry in 18 successively hospitalized term neonates with ABO haemolytic disease of newborn, 21 term neonates with jaundice without haemolysis needing phototherapy, and 47 term healthy neonates were compared. The COHb levels were higher in the group of neonates with ABO haemolytic disease of newborn compared to the COHb levels in jaundiced neonates without haemolysis and healthy neonates (median COHb 2.4 % (IQR 1.3) vs. 1.3 % (IQR 0.8) and 1.3 % (IQR 0.2), respectively) and the differences were statistically significant. The results of multivariate analysis showed that haemolysis had the biggest, statistically significant effect on the COHb values compared to other variables included in the model. Receiver operating characteristic (ROC) curve analysis was performed to evaluate a diagnostic value of COHb measured with CO-oximetry as a test for detecting haemolysis in neonates. According to the receiver operating characteristic area under the curve (ROC AUC), the COHb measured with CO-oximetry is a good diagnostic test for confirming haemolysis in neonates, since ROC AUC was more than 0.9, and the sensitivity and specificity of the optimal cut off COHb value, which was in our study set as ≥1.7 %, was very high. In the second, retrospective part of the study, COHb values measured with CO-oximetry in 8 term neonates with neonatal sepsis, 37 term neonates with respiratory distress, 16 term neonates with haemolytic disease of newborn, and 76 healthy neonates were compared. Also, the retrospective analysis of the COHb values measured with CO-oximetry showed statistically significantly higher COHb values in neonates with haemolysis (median COHb was 2.4 % (IQR 1.3)), while the values of COHb between neonates with sepsis and respiratory distress (median COHb 1.4 % (IQR 0.5) and 1.4 % (IQR 0.4), respectively) and healthy neonates (median COHb 1.3 % (IQR 0.3)) were not statistically different, although the median COHb values were slightly higher in neonates with sepsis and respiratory distress compared to healthy neonates. Furthermore, the value of COHb measured with CO-oximetry as a diagnostic test for detecting haemolysis in neonates was evaluated with ROC curve analysis of retrospective obtained data. The ROC curve analysis confirmed the result of the prospective study, since the ROC AUC of retrospective data was high (>0.9), and the specificity and sensitivity of the optimal cut off COHb value, which was the same as in prospective study (≥1.7 %), were high, even though the neonates with sepsis and respiratory distress were included in the analysis. The correlation between the COHb values and postnatal age was negative, since COHb values are declining in the first days after the birth. The mean COHb value in healthy neonates was 1.35 % on the 1st day after the birth, 1.26 % on the 7th day after the birth, and 1.01 % on the 14th day after the birth.
In the discussion, the results of the research are compared with the results of other studies which evaluated the production of CO and COHb values, determined with other laboratory methods, in different groups of neonates. The COHb values measured with CO-oximetry in neonates with haemolysis are similar or slightly higher than the COHb values measured with gas chromatography. The comparison of the COHb values measured with oximetry or gas chromatography in healthy neonates and neonates with jaundice without haemolysis showed similar trend, probably because the oximetry method doesn't correct the COHb value for inhaled air CO. Absence of the correction of COHb value according to the environmental levels of CO seems to be the most important limitation of CO-oximetry, as well as of this study. Nevertheless, the results of the research show that COHb measured with CO-oximetry is a good diagnostic test for detecting haemolysis in neonates, since it performs well in discriminating neonates with and without haemolysis. The results of this research open new possibilities for further work, such as an evaluation of the COHb values measured with CO-oximetry in preterm neonates and an evaluation of the COHb values measured with CO-oximetry in the prediction of icterus and its severity.
The results of the study confirm that COHb measured with CO-oximetry is a good diagnostic test for detecting haemolysis in term neonates, and the specificity and the sensitivity of the cut off COHb value of ≥1.7 % are very high. The influence of other clinical factors that may induce HO-1 and influence the COHb values is small and statistically insignificant, therefore the discrimination of the test is high despite the presence of sepsis or respiratory distress, factors that may induce HO-1. The results of the study represent the base for clinical use of COHb measured with CO-oximetry as a diagnostic test for detecting haemolysis in term neonates.
|
