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. People at increased risk include first-degree relatives of a patient with type 1 diabetes and people with high genetic risk. Examples are carriers of certain HLA (human leukocyte antigen) haplotypes [1]. Another common situation is clarifying the diagnosis in adults with type 2 (or unclear) diabetes who have features suggestive of type 1 diabetes. Such features are young age at diagnosis, unintentional weight loss and ketoacidosis at onset. Rapid intensification of diabetes treatment (for example adding a basal insulin within the first 5 years) also counts [2].

With the incidence of type 1 diabetes rising worldwide, some countries have introduced population screening programs:

• Italy — in 2023 adopted the first national screening law for type 1 diabetes and celiac disease in children aged 1–17 years;
• Fr1da — in Bavaria;
• GPPAD — in Europe;
• Type1Screen — in Australia;
• ASK and CASCADE — in the USA [3] [4].

Relatives of people with type 1 diabetes can also be tested through TrialNet, in the USA and in other countries. Early identification of autoantibodies allows educating the family about the signs of the disease and prevention of ketoacidosis at onset. It also opens the possibility of taking part in a clinical trial or even access to therapies that delay progression, such as teplizumab [4]. Teplizumab is given as a daily infusion for 14 days under medical supervision, because it temporarily lowers the lymphocyte count and can reactivate some viruses, sometimes (rarely) with serious consequences [5] [6].

If you have type 1 diabetes, your child's risk of developing this disease is significantly higher than in the general population. This is why autoantibody testing is useful [7]. 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. That is when most type 1 diabetes-specific autoantibodies appear [8].

After a negative test, children from families with type 1 diabetes are usually retested only once before the age of 18 [7]. Anyone with one confirmed autoantibody repeats testing at intervals of 6 months to 3 years, depending on age [2]. The main benefit of family testing is the prevention of ketoacidosis at onset. Children who are tested and then monitored are less likely to be in a critical state at diagnosis. Testing and monitoring reduce the risk of ketoacidosis, but they do not remove it completely. If your child develops vomiting, difficulty breathing or drowsiness, take them to the emergency room right away. The decision to test is made after a careful discussion with your doctor. This is because testing also requires preparing the family psychologically for a possible positive result [7].

The four standard autoantibodies in type 1 diabetes are:

• GADA — anti-glutamic acid decarboxylase;
• IA-2A — anti-islet antigen 2, also called anti-tyrosine phosphatase;
• IAA — anti-insulin;
• ZnT8A — anti-zinc transporter 8 [9].

Several laboratory techniques exist for detecting them. The traditional methods, used for decades, were radioimmunoassay (RIA), which uses radioactive markers, and ELISA, based on colorimetric enzymatic reactions [9].

The newer techniques have improved the precision and safety of testing. Electrochemiluminescence (ECL) and the luminescent immunoprecipitation system (LIPS) detect even low titers with high specificity. They are useful especially in adults, where the autoimmune signal may be weaker. There are also better methods, such as the "bridge assay", in which the antibody binds two labeled antigen molecules at the same time. This increases specificity [10]. Optimal results are obtained in laboratories that take part in the Islet Autoantibody Standardization Program (IASP), an international program that periodically verifies the performance of the methods used [10].

Methods with high specificity include ECL (electrochemiluminescence) and LIPS (luminescent immunoprecipitation system). ECL is currently considered one of the best-performing methods for detecting low-titer autoantibodies. It is useful especially in adults with suspected LADA, where the autoimmune signal may be weak [10]. RIA remains the historical reference method, but the use of radioisotopes makes it less practical for modern laboratories [9].

Regardless of the technology used, the most important criterion is whether the laboratory takes part in the Islet Autoantibody Standardization Program (IASP). This program periodically evaluates test performance using standardized samples [10]. Commercial ELISA methods are more accessible and can give equally good results if the laboratory takes part in IASP with good results [10]. The first positive test must be confirmed by a second test within 3 months, ideally in a laboratory that takes part in IASP. Weakly positive results may be transient or false positive [10].

IAA (anti-insulin autoantibodies) are antibodies directed against your own insulin molecule. The immune system produces them even before beta cells are destroyed. In young children they are often the first autoantibody to appear [11]. The problem is that the body can recognize insulin injected subcutaneously as a foreign protein. This happens even with human insulin identical to the one your pancreas produces. Thus, in the first days or weeks of treatment, antibodies against the exogenous insulin commonly appear [9].

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. The test cannot distinguish whether the immune response is directed against your own insulin or against the injected one [9]. In these situations, your doctor relies on the other autoantibodies (GADA, IA-2A, ZnT8A), which are not influenced by insulin treatment [11].

A weakly positive result means a value slightly above the positivity threshold set by the laboratory, without a clearly elevated titer. This type of result may reflect real autoimmunity at an early stage, a transient phenomenon, or technical interference from the method. Transient results are seen especially in young children, in whom some isolated positive tests return to negative within a few months [12]. That is why any positive test should be confirmed by a second test within approximately three months [13].

The practical approach, however, depends on the context. If you have suggestive symptoms or other risk factors, your doctor may decide on additional investigations. Examples are a fasting blood glucose, glycated hemoglobin (HbA1c) or, more rarely, an oral glucose tolerance test [13]. If the weakly positive result is isolated and is not confirmed, continued monitoring nonetheless remains necessary, because autoantibodies may appear later. Keep in mind: a weakly positive result does not mean a diagnosis of type 1 diabetes. It is only a signal that requires observation and interpretation together with your doctor, as the figure below shows [10].

A high autoantibody titer 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 [14]. The higher the titer, the greater the probability that the autoantibody is persistent, not just transient [14].

The risk of progression depends, however, not only on the titer, but also on other factors:

• the number of autoantibodies present;
• their specificity — IA-2A and ZnT8A are associated with faster progression than isolated GADA;
• the age at seroconversion — the younger, the higher the risk;
• the presence of genetic risk factors, such as certain HLA haplotypes [1] [15].

A high titer of an antibody with high specificity, such as IA-2A in a young child, indicates a significant risk and justifies more frequent metabolic monitoring [15].

Yes. The phenomenon is called seroconversion and means the appearance of the first autoantibody, previously absent. Seroconversion can occur at any time in childhood. There are 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) [8] [12]. In adults, seroconversion remains possible, especially in the context of other autoimmune diseases [11].

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), after a negative result, the test must be repeated at the intervals established by your doctor [8]. This approach detects seroconversion as close as possible to the time when it occurs. Targeted metabolic monitoring can then start 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). The same prick can be used in two ways. The first is liquid capillary blood, collected in a very thin tube and sent to the laboratory. The second is the "dried blood spot" (DBS) variant: a few drops are placed on a special paper card, left to dry and then sent off [16]. Several international programs use these methods, such as Fr1da in Bavaria (liquid capillary blood), T1Detect (DBS) and Type1Screen (DBS, in Australia) [17].

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 a blood draw from a vein and at a low cost. This makes it possible to test a larger number of children [17]. 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. This is because the capillary test can give weakly positive or false positive results [16]. If the result is confirmed, a complete metabolic evaluation follows to establish the stage of type 1 diabetes.