Testing for type 1 diabetes-specific autoantibodies

The main situation in which testing for autoantibodies against beta cells is recommended is screening for asymptomatic type 1 diabetes in people at increased risk. Considered at increased risk are first-degree relatives of a patient with type 1 diabetes, as well as people with high genetic risk, for example carriers of certain HLA haplotypes [1]. Another frequent situation is clarifying the diagnosis in adults labelled with type 2 diabetes (or unclear), but who have features suggestive of type 1 diabetes: young age at diagnosis, unintentional weight loss, ketoacidosis at onset, or rapid progression of antidiabetic treatment (for example adding a basal insulin within the first 5 years) [2].

Against the background of the rising incidence of type 1 diabetes worldwide, some countries have introduced population screening programmes: in 2023 Italy adopted the first national screening law for type 1 diabetes and coeliac disease in children aged 1-17 years, and other important programmes are Fr1da in Bavaria, GPPAD in Europe, Type1Screen in Australia, and TrialNet, ASK and CASCADE in the USA [3] [4]. Early identification of autoantibodies allows education about the signs of the disease, prevention of ketoacidosis at onset, and the possibility of taking part in a clinical trial or even access to therapies that delay progression, such as teplizumab [4].

If you have type 1 diabetes, your child's risk of developing this disease is significantly higher compared with the general population, which is why autoantibody testing is useful [5]. Testing can begin at any time in childhood. The most likely periods for detecting seroconversion (the appearance of the first autoantibody) are around the age of two years and at 5-6 years, because that is when most type 1 diabetes-specific autoantibodies appear [6].

In children under 3 years, testing is recommended every 6 months for 3 years, then annually for another 3 years, and in children aged 3-18 years annual testing is recommended [5]. The main benefit of family testing is the prevention of ketoacidosis at onset, because children who are tested and then monitored less often reach a critical state at the moment of diagnosis. The decision to undergo testing is made after a careful discussion with your doctor, because it also involves psychological preparation of the family for the possibility of a positive result [5].

There are several laboratory techniques for detecting the four standard autoantibodies in type 1 diabetes: GADA (anti-glutamic acid decarboxylase), IA-2A (anti-tyrosine phosphatase 2), IAA (anti-insulin) and ZnT8A (anti-zinc transporter 8) [7]. The classic methods, used for decades, were radioimmunoassay (RIA), which uses radioactive markers, and ELISA, based on colorimetric enzymatic reactions [7].

The newer techniques have improved the precision and safety of testing. Electrochemiluminescence (ECL) and the luminescent immunoprecipitation system (LIPS) detect even low titres with high specificity, being useful especially in adults, where the autoimmune signal may be weaker. There are also superior methods, of the "bridge assay" type, which use two epitopes to recognise the antibody, which increases specificity [8]. Optimal results are obtained in laboratories that take part in the Islet Autoantibody Standardization Program (IASP), an international programme that periodically verifies the performance of the methods used [8].

The most accurate methods are those with high specificity, such as ECL (electrochemiluminescence) and LIPS (luminescent immunoprecipitation system). ECL is currently considered one of the best-performing methods for detecting low-titre autoantibodies, being useful especially in adults with suspected LADA, where the autoimmune signal may be weak [8]. RIA remains the historical reference method, but the use of radioisotopes makes it less practical for modern laboratories [7].

Regardless of the technology used, the most important criterion is whether the laboratory takes part in the Islet Autoantibody Standardization Program (IASP), which periodically evaluates test performance using standardised samples [8]. ELISA and other commercial methods of lower quality are more accessible (financially or logistically) and can give correct results if the laboratory has been IASP-validated. The first positive test must be confirmed by a second test within 3 months, ideally in an IASP-certified laboratory, because weakly positive results may be transient or false positive [8].

IAA (anti-insulin autoantibodies) are antibodies directed against your own insulin molecule and are produced by the immune system even before the destruction of beta cells; in young children they are often the first autoantibody to appear [9]. The problem arises because any insulin injected subcutaneously is recognised by the body as a foreign protein, even if human insulin identical to that produced by your pancreas is used. Thus, in the first days or weeks of treatment you will produce antibodies against the exogenous insulin [7].

For this reason, IAA is useful only in people who have not yet received insulin. In a patient with type 1 diabetes already on insulin therapy, a positive IAA test no longer has diagnostic value, because it cannot distinguish whether the autoimmune response is directed against your own insulin or against the injected one [7]. In these situations, your doctor relies on the other autoantibodies (GADA, IA-2A, ZnT8A), which are not influenced by insulin treatment [9].

A weakly positive result means a value situated slightly above the positivity threshold established by the laboratory, without a clearly elevated titre. This type of result may reflect a real autoimmunity in an early stage, a transient phenomenon (especially in young children, in whom some isolated positive tests return to negative within a few months), or a technical interference of the method [10]. That is why confirmation of any positive test by a second test is recommended, within an interval of approximately three months [11].

The practical approach, however, depends on the context. If you have suggestive symptoms or other risk factors, your doctor may decide on additional investigations, such as a fasting blood glucose, glycated haemoglobin (HbA1c) or, more rarely, an oral glucose tolerance test [11]. If the weakly positive result is isolated and is not confirmed, continued monitoring nonetheless remains necessary, because autoantibodies may appear later. Important to remember: a weakly positive result does not mean a diagnosis of type 1 diabetes, but only a signal that requires observation and interpretation together with your doctor [8].

A high autoantibody titre reflects a strong immune response and is associated with a higher risk of progression to symptomatic type 1 diabetes (stage 3). For each antibody, laboratories report not only positivity (yes/no), but also a quantitative value, which allows risk stratification [12]. The higher the titre, the greater the probability that the autoantibody is persistent, not just transient [12].

The risk of progression depends, however, not only on the titre, but also on other factors: the number of autoantibodies present, their specificity (IA-2A and ZnT8A are associated with faster progression compared with isolated GADA), the age at seroconversion (the younger, the higher the risk) and the presence of genetic risk factors, such as certain HLA haplotypes [13] [1]. A high titre of an antibody with high specificity, such as IA-2A in a young child, indicates a significant risk and justifies more frequent metabolic monitoring [13].

Yes. The phenomenon is called seroconversion and means the appearance of the first autoantibody, previously absent. Large-scale paediatric studies, such as TEDDY, BABYDIAB and DAISY, have shown that seroconversion can occur at any time in childhood, with two main peaks: one around the age of 2 years (more frequent for IAA) and another around the age of 5-6 years (more frequent for GADA) [10] [6]. In adults, seroconversion remains possible, especially in the context of other autoimmune diseases [9].

Precisely because autoantibodies can appear at any moment, a single negative test does not definitively exclude the future risk of type 1 diabetes, but only reduces it. In a person at increased risk (first-degree relative, genetic risk factors), a negative result must be repeated at the intervals established by your doctor [6]. This approach allows identification of seroconversion as close as possible to the moment when it occurs and the initiation of targeted metabolic monitoring, before the disease becomes symptomatic.

Yes, there are screening tests from capillary blood, collected through a simple prick in the fingertip (or in the heel in infants). Starting from the same prick there are two variants: liquid capillary blood, collected in a very thin tube and sent to the laboratory, or the "dried blood spot" (DBS) variant, in which a few drops are placed on a special paper card, left to dry and then sent off [14]. Several international programmes use these methods, such as Fr1da in Bavaria (liquid capillary blood), T1Detect (DBS) and Type1Screen (DBS, in Australia) [15].

The main advantage of capillary testing is accessibility: it can be done at the family doctor's office or even at home, with a small prick instead of venous puncture and at a low cost, which allows testing of a larger number of children [15]. The limitation is that any positive result must be confirmed by a second test, collected as a venous sample, in a laboratory that complies with IASP standards, because the capillary test can give weakly positive or false positive results [14]. If the result is confirmed, complete metabolic evaluation follows to establish the stage of type 1 diabetes.