A look at the strategies that Improve accuracy and efficiency in diagnosis of Celiac Disease
Public awareness of celiac disease (CD) has been growing for years. This awareness, combined with additional interest from the scientific community, has shed important light on this previously obscure autoimmune disorder. A 2012 study in the American Journal of Gastroenterology, for example, found that 82 percent of participants who were diagnosed as having Celiac Disease (either double-positive serology or a reported diagnosis of CD by a doctor or other health-care professional and being on a Gluten Free Diet) were previously unaware that they suffered from it. Today, it is estimated that roughly one percent of the population is affected by CD. If only 18 percent are diagnosed, that implies that there are approximately two and a half million undiagnosed celiac cases in the United States alone.
High volumes of mis- or undiagnosed CD cases – combined with increased public awareness of the disease – are creating significant new demand for CD testing. In order to effectively serve these populations, laboratories will need to increase the accuracy and efficiency with which they diagnose CD. There are two primary ways to approach this problem: improving the sensitivity, specificity and speed of their assays and diagnostic procedures; and through increasing the efficiency of laboratory processes to handle the expected increase in sample volume.
Assays and Diagnostics
The first diagnostic antigen to be associated with a CD assay was gliadin. Persons with CD are intolerant to gliadin and other proteins found in gluten. When they consume them, the proteins are modified by tissue transglutaminase (tTG) enzymes in their small intestines, and the body perceives both the tTG and the gluten proteins as antigens that are foreign to the body. The body then produces antibodies of the IgG and IgA isotypes to neutralize the perceived threat. These immunoglobulins come in two primary types: anti-
tTG, which target tTG, and anti-EMA, which target the endomysium, a tTG-containing layer of connective muscle tissue.
The first – and, ideally, only – necessary step for a conclusive celiac diagnosis is a serological assay that measures either anti-EMA or anti-tTG immunoglobulins. Positive results from these tests are usually followed by a confirmatory jejunal (small intestine) biopsy, so high specificity is particularly critical for screening assays. Since CD reportedly affects approximately one percent of the population, even a small decrease in specificity can cause a significant jump in the false positive rate and possibly unneeded biopsies.
Clinicians have considered EMA assays to be the gold standard for celiac diagnosis specificity for decades, but today EMAs share that distinction with tTG assays. This is due in large part to the development of human recombinant tTG assays, as its highly pure and conformationally correct antigen can achieve much higher specificity and sensitivity than that for previous generations. But while human recombinant tTG and EMA assays may be equals with respect to sensitivity and specificity, the efficiency of the diagnostic processes surrounding the two assays is not.
Laboratory Processes
The potential for automation in the tTG and EMA analytical processes is very different. While EMA (based on indirect immunofluorescence) is labor intensive and costly, much of the anti tTG analytical workload can be automated without impacting diagnostic accuracy. Since they have achieved parity with EMA for specificity and sensitivity, tTG assays can now offer clinicians a much more cost-effective solution for confirming a CD diagnosis.
Although a number of automated analytical instruments can run tTG assays, certain features are worth noting. Among the most important features for many labs is continuous random access. Instead of waiting for a sufficient number of samples to fill a plate, samples are able to be run as they come in. Greater flexibility is possible without added costs, which simultaneously decreases laboratory turnaround time (TAT) and can increase profitability.
The ability to self-dilute samples is also an important feature for any tTG analysis instrument. Anti-tTG samples need to be diluted before analysis, and onboard dilution increases the efficiency of the overall process by reducing the need for manual labor.
Finally – and perhaps most important – the analyzer must be able to identify samples that indicate potential IgA deficiency. Selective IgA deficiency (SIgAD) is a very common primary immunodeficiency characterized by a total IgA serum level below 0.05 g/L. To account for this, analytical instruments must be able to detect IgA levels that are both markedly above (indicating CD) and markedly below (indicating the potential for SIgAD) normal serum IgA levels. While tTG assays cannot diagnose SIgAD, they can be useful in identifying candidates for further testing. The fact that SIgAD subjects are 10 to 20 times more likely to have CD than the general population makes this capability especially important, since their low serum IgA levels could be misinterpreted as a false negative for CD if the analyzer is not sensitive enough.
Conclusion
There are many theories to explain why CD rates have seen such a dramatic increase over the past few decades. Hypotheses range from changes in society’s hygiene habits to the simple fact that diagnostic tools are more widely applied today. What is clear, though, is that demand for laboratory-based celiac disease diagnostics will continue to increase in the future.
Over the next few years, laboratories that offer celiac diagnostics will need to transition to highly accurate and efficient assays (and instruments) to better respond to increased demand. Human recombinant tTG assays, working in concert with highly automated laboratory instruments, offer diagnostic laboratories a unique opportunity to improve operations and profitability while enabling the healthcare professionals they support to more effectively diagnose celiac disease.
