Screening for inherited metabolic disease in newborn infants using tandem mass spectrometry

Although individually rare, inborn errors of metabolism represent a potentially preventable cause of death and disability. Screening for phenylketonuria (birth prevalence 10 per 100 000) was introduced in the United Kingdom over 30 years ago. It has proved successful in preventing severe mental retardation. The development of tandem mass spectrometry enables a wide variety of additional compounds to be assayed on the dried blood spots routinely collected from newborn infants.¹ The combined birth prevalence of disorders, excluding phenylketonuria, which could be detected by screening is about 20 per 100 000. Of these, medium chain acyl CoA dehydrogenase deficiency is one of the most important. However, despite experience of screening over a million infants, many questions about screening for this disorder remain unanswered.

In the United Kingdom between 5 and 11 per 100 000 live born infants have medium chain acyl CoA dehydrogenase deficiency, which is about 35 to 70 children each year.² This recessively inherited disorder classically presents during infancy and early childhood with a severe illness characterised by encephalopathy and hypoglycaemia. This is usually precipitated by a minor febrile illness, particularly gastroenteritis, and fasting. Of those presenting clinically, up to a quarter will die and about a third of survivors will have irreversible neurological damage.³,⁴ In a significant proportion there is a history of previous sudden unexplained death or encephalopathy in a sibling.⁴ However, the presentation varies widely, with some individuals not presenting until they are adults and an unknown number remaining undiagnosed or asymptomatic. In people of northern European descent, over 80% of clinically diagnosed patients are homozygous for one mutation—G985A. Simple heterozygotes have no symptoms. The mainstay of treatment is a high carbohydrate diet, orally or intravenously during fasting or intercurrent infection.⁵ This seems to be effective, with few of the 162 children reported in the two largest series having further episodes of encephalopathy.³,⁶

The outcome for siblings diagnosed prospectively also seems good.⁶ Given this clinical course and response to treatment, medium chain acyl CoA dehydrogenase deficiency has been identified as a potential candidate for early detection through newborn screening.⁷,⁸

Several centres outside the United Kingdom have introduced newborn screening for medium chain acyl CoA dehydrogenase deficiency by using tandem mass spectrometry to measure acyl carnitines.⁹ Carpenter et al have recently reported identifying 11 babies with definite medium chain acyl CoA dehydrogenase deficiency among 275 000 screened, a birth prevalence of 4 per 100 000¹⁰—which was lower than expected. Their publication highlights many of the questions and uncertainties that remain about performance and outcome.

Differences in the choice of metabolite as well as in thresholds used to define a positive result limit direct comparison of test performance between centres. A further issue is the criteria used to confirm a diagnosis of medium chain acyl CoA dehydrogenase deficiency. In one study from the United States 62 infants were considered to have medium chain acyl CoA dehydrogenase deficiency solely on the basis of “pathological acyl carnitine profiles.”⁹ By contrast, Carpenter et al applied explicit independent diagnostic criteria to 23 infants with positive screening results and diagnosed definite medium chain acyl CoA dehydrogenase deficiency in 11, with one further probable mild case.¹⁰ A striking finding in this report is that of the remaining 11 babies who screened positive but did not meet the diagnostic criteria for medium chain acyl CoA dehydrogenase deficiency (false positives), four died in the neonatal period. This is consistent with observations that infants and young children who are ill for any reason may have abnormal patterns of acyl carnitines.¹¹ In a retrospective study based on 100 600 dried blood spots, all but one of those with false positive results were preterm babies.¹²

The false negative rate is difficult to determine, as none of the prospective studies included a rigorous scheme to identify those who might have escaped detection. Babies who have rapidly become carnitine depleted may be missed. It is already clear that newborn screening identifies some individuals whose history is not known and who may be treated unnecessarily. In both the Australian and the US study the frequency of the common mutation was lower than expected, and the proportion of A985G heterozygotes higher.⁹,¹⁰

To be maximally effective screening needs to be done and acted on very soon after birth. Up to one third of those with medium chain acyl CoA dehydrogenase deficiency have been reported to present in the first three days of life,⁴ and this may be an underestimate since the diagnosis is easily missed. Appropriate samples need to be collected from all sick newborn infants, including those who die. Detection and timely diagnosis will also depend on the age at which screening is undertaken. In all the prospective studies reported to date the screening specimen was collected between the second and third day of life, considerably earlier than in the United Kingdom, where this is generally collected between the sixth and tenth day. However, bringing forward the age at screening in the United Kingdom may influence test performance for other conditions that share the same screening infrastructure. Ensuring that screening improves outcome will depend on the timely delivery of high quality clinical services to babies and their families. This is a considerable challenge, especially since there is a shortage of clinicians with training in paediatric metabolic medicine. Judgments about the effectiveness of screening need to be informed by prospective data on mortality, neurological outcome, and cognitive function. Additionally the impact of screening and treatment on the families of infants with true, borderline, and false positive diagnoses needs to be taken into account. Despite increasing experience of screening for medium chain acyl CoA dehydrogenase deficiency, there has been no report of a systematic follow up of longer term outcome in affected infants detected by screening.

Randomised trials of screening for rare disorders are difficult, but observational data from largescale prospective collaborative studies can provide information on test and programme performance and clinical outcome to guide policy decisions.⁷,¹³ These require a coordinated strategy and resources, which have proved difficult to secure but are vital if newborn screening programmes are to serve infants and their families and not simply be technology led.

The United Kingdom health technology assessment programme had the foresight to commission systematic reviews in this field in the early 1990s⁷,⁸ and originally gave high priority to evaluating newborn screening for medium chain acyl CoA dehydrogenase deficiency. The subsequent failure to fund this primary research has left many stakeholders disillusioned and frustrated.¹⁴ In the United Kingdom many newborn screening laboratories are introducing tandem mass spectrometry to screen for phenylketonuria. It now seems probable that acyl carnitines will be added, but once again, a screening technology looks set to be driven by enthusiasm and opinion rather than evidence.