NCGS and celiac disease cannot be separated in diagnosis because many gastrointestinal and non-gastrointestinal symptoms are similar in both diseases, and there are people with celiac disease having negative serology (absence of specific celiac disease antibodies in serum) or without villus atrophy. There is no test capable of eliminating a diagnosis of a celiac disease, but such a diagnosis is unlikely without confirming HLA-DQ2 and/or HLA-DQ8 haplotypes.The prevalence of undiagnosed celiac disease has increased fourfold during the past half-century, with most cases remaining unrecognized, undiagnosed and untreated, leaving celiac patients with the risk of long-term complications. Some people with NCGS may indeed have celiac disease. A 2015 systematic review found that 20% of people with NCGS presenting with HLA-DQ2 and/or HLA-DQ8 haplotypes, negative serology, and normal histology or duodenal lymphocytosis had celiac disease.The presence of autoimmune symptoms in people with NCGS suggests the possibility of undiagnosed celiac disease. Autoimmune diseases typically associated with celiac disease are diabetes mellitus type 1, thyroiditis, gluten ataxia, psoriasis, vitiligo, autoimmune hepatitis, dermatitis herpetiformis, primary sclerosing cholangitis, and others.To evaluate the possible presence of celiac disease, specific serology and duodenal biopsies are required while the person is still on a diet that includes gluten. Serological markers
Serological CD markers (IgA tissue transglutaminase [tTGA], IgA endomysial [EmA] and IgG deamidated gliadin peptide [DGP] antibodies) are always negative in those with NCGS; in addition to specific IgA autoantibody levels, it is necessary to determine total IgA levels.
Gastrointestinal
Gastrointestinal symptoms may include any of the following: abdominal pain, bloating, bowel habit abnormalities (either diarrhea or constipation), nausea, aerophagia, flatulence, gastroesophageal reflux disease, and aphthous stomatitis. Extraintestinal
NCGS can cause a wide range of extraintestinal symptoms, which can be the only manifestation of NCGS in absence of gastrointestinal symptoms. These include any of the following: headache, migraine, "foggy mind", fatigue, fibromyalgia, joint and muscle pain, leg or arm numbness, tingling of the extremities, dermatitis (eczema or skin rash), atopic disorders such as asthma, rhinitis, other allergies, depression, anxiety, iron-deficiency anemia, folate deficiency, or autoimmune diseases. NCGS is also linked to a wide spectrum of neurological and psychiatric disorders, including  ataxia, schizophrenia, epilepsy, peripheral neuropathy, encephalopathy, vascular dementia, eating disorders, autism, attention deficit hyperactivity disorder (ADHD), hallucinations (so-called "gluten psychosis"),  and various movement disorders (restless legs syndrome, chorea, parkinsonism, Tourette syndrome, palatal tremor,  myoclonus, dystonia, opsoclonus myoclonus syndrome, paroxysms, dyskinesia, myorhythmia, myokymia).Above 20% of people with NCGS have IgE-mediated allergy to one or more inhalants, foods, or metals, among which most common are mites, graminaceae, parietaria, cat or dog hair, shellfish, and nickel. Approximately 35% of patients suffer other food intolerances, mainly lactose intolerance. Causes
The pathogenesis of NCGS is not yet well understood, but the activation of the innate immune system, the direct cytotoxic effects of gluten, and probably the cytotoxicity of other wheat molecules are implicated. Besides gluten, other components in wheat, rye, barley, and their derivatives, including amylasetrypsin inhibitors (ATIs) and FODMAPs, may  cause symptoms.