The cause of Felty's syndrome is unknown, but it has been found to be more common in those with chronic rheumatoid arthritis. Some patients have Human Leukocytic Antigen (HLA-DR4) in their serum. This syndrome is mostly present in people having extra articular manifestations of rheumatoid arthritis. People with this syndrome are at risk of infection because they have a low white blood cell count.

If rheumatoid arthritis is present and other symptoms occur that are not commonly found within RA itself, such as a palpable spleen, further testing should be done. A palpable spleen is not always a clinical significance, therefore CT scan, MRI, or ultrasound can be administered in order to help diagnose the condition. According to Poulin et al, dimensional guidelines for diagnosing splenomegaly are as follows:

- Moderate if the largest dimension is 11-20 cm

- Severe if the largest dimension is greater than 20 cm

Clinical features along with the familial tendency may be enough to make a diagnosis. Genetic testing may also be used.

Because CAPS is extremely rare and has a broad clinical presentation, it is difficult to diagnose, and a significant delay exists between symptom onset and definitive diagnosis. There are currently no clinical or diagnostic criteria for CAPS based solely on clinical presentation. Instead, diagnosis is made by genetic testing for "NLRP3" mutations. Acute phase reactants and white blood cell count are usually persistently elevated, but this is aspecific for CAPS.

The blood count typically shows decreased numbers of blood cells—including a decreased amount of circulating red blood cells, white blood cells, and platelets.

The bone marrow may show hemophagocytosis.

The liver function tests are usually elevated. A low level of the protein albumin in the blood is common.

The serum C reactive protein, erythrocyte sedimentation rate, and ferritin level are markedly elevated. In children, a ferritin above 10000 is very sensitive and specific for the diagnosis of HLH, however, the diagnostic utility for ferritin is less for adult HLH patients.

The serum fibrinogen level is usually low and the D-dimer level is elevated.

The sphingomyelinase is elevated.

Bone marrow biopsy shows histiocytosis.

The current (2008) diagnostic criteria for HLH are

1. A molecular diagnosis consistent with HLH. These include the identification of pathologic mutations of PRF1, UNC13D, or STX11.

OR

2. Fulfillment of five out of the eight criteria below:

- Fever (defined as a temperature >100.4 °F, >38 °C)

- Enlargement of the spleen

- Decreased blood cell counts affecting at least two of three lineages in the peripheral blood:

- Haemoglobin <9 g/100 ml (in infants <4 weeks: haemoglobin <10 g/100 ml) (anemia)

- Platelets <100×10/L (thrombocytopenia)

- Neutrophils <1×10/L (neutropenia

- High blood levels of triglycerides (fasting, greater than or equal to 265 mg/100 ml) and/or decreased amounts of fibrinogen in the blood (≤ 150 mg/100 ml)

- Ferritin ≥ 500 ng/ml

- Haemophagocytosis in the bone marrow, spleen or lymph nodes

- Low or absent natural killer cell activity

- Soluble CD25 (soluble IL-2 receptor) >2400 U/ml (or per local reference laboratory)

In addition, in the case of familial HLH, no evidence of malignancy should be apparent.

It should be noted that not all five out of eight criteria are required for diagnosis of HLH in adults, and a high index of suspicion is required for diagnosis as delays results in increased mortality. The diagnostic criteria were developed in pediatric populations and have not been validated for adult HLH patients. Attempts to improve diagnosis of HLH have included use of the HScore, which can be used to estimate an individual's risk of HLH.

The diagnosis is based on observing the patient and finding the constellation of symptoms and signs described above. A few blood tests help, by showing signs of long standing inflammation. There is no specific test for the disease, though now that the gene that causes the disease is known, that may change.

Routine laboratory investigations are non specific: anaemia, increased numbers of polymorphs, an elevated erythrocyte sedimentation rate and elevated concentrations of C-reactive protein are typically all the abnormalities found. Lumbar puncture shows elevated levels of polymorphs (20-70% of cases) and occasionally raised eosinophil counts (0-30% of cases). CSF neopterin may be elevated.

The X ray changes are unique and charactistic of this syndrome. These changes include bony overgrowth due to premature ossification of the patella and the long bone epiphyses in very young children and bowing of long bones with widening and shortening periosteal reaction in older ones.

Audiometry shows a progressive sensineural deafness. Visual examination shows optic atrophy and an increase in the blind spot. CT is usually normal but may show enlargement of the ventricles. MRI with contrast may show enhancement of leptomeninges and cochlea consistent with chronic meningitis. EEG shows is non specific with slow waves and spike discharges.

Polymorphs tend to show increased expression of CD10.

Still's disease does not affect children under 6 months old.

Hyperimmunoglobulin D syndrome in 50% of cases is associated with mevalonate kinase deficiency which can be measured in the leukocytes.

Since interleukin 1β plays a central role in the pathogenesis of the disease, therapy typically targets this cytokine in the form of monoclonal antibodies (such as canakinumab), binding proteins/traps (such as rilonacept), or interleukin 1 receptor antagonists (such as anakinra). These therapies are generally effective in alleviating symptoms and substantially reducing levels of inflammatory indices. Case reports suggest that thalidomide and the anti-IL-6 receptor antibody tocilizumab may also be effective.

Acne treatment may require oral tetracycline antibiotics or isotretinoin. Treatments directed at tumor necrosis factor (TNF) (infliximab, etanercept) and interleukin-1 (anakinra) have shown a good response in resistant arthritis and pyoderma gangrenosum. Other traditional immunosuppressant treatments for arthritis or pyoderma gangrenosum may also be used.

Canakinumab has been approved for treatment of HIDS and has shown to be effective. The immunosuppressant drugs etanercept and anakinra have also shown to be effective. Statin drugs might decrease the level of mevalonate and are presently being investigated. A recent single case report highlighted bisphosphonates as a potential therapeutic option.

The 5 year survival has been noted as 89% in at least one study from France of 201 patients with T-LGL leukemia.

It is not known how mevalonate kinase mutations cause the febrile episodes, although it is presumed that other products of the cholesterol biosynthesis pathyway, the prenylation chains (geranylgeraniol and farnesol) might play a role.

Clonal rearrangements of the T-cell receptor (TCR) genes are a necessary condition for the diagnosis of this disease. The gene for the β chain of the TCR is found to be rearranged more often than the γ chain. of the TCR.

Familial dysautonomia is inherited in an autosomal recessive pattern, which means 2 copies of the gene in each cell are altered. If both parents are shown to be carriers by genetic testing, there is a 25% chance that the child will produce FD. Prenatal diagnosis for pregnancies at increased risk for FD by amniocentesis (for 14–17 weeks) or chorionic villus sampling (for 10–11 weeks) is possible.

Sudden cessation of high-dose corticosteroids, opioids, barbiturates, benzodiazepines, caffeine or alcohol can induce myalgia in many respects.

Genetic testing is performed on a small sample of blood from the tested individual. The DNA is examined with a designed probe specific to the known mutations. The accuracy of the test is above 99%. Dr. Anat Blumenfeld of the Hadasah Medical center in Jerusalem identified chromosome number 9 as the responsible chromosome.

Liver transplantation has proven to be effective for ATTR familial amyloidosis due to Val30Met mutation.

Alternatively, a European Medicines Agency approved drug Tafamidis or Vyndaqel now exists which stabilizes transthyretin tetramers comprising wild type and different mutant subunits against amyloidogenesis halting the progression of peripheral neuropathy and autonomic nervous system dysfunction.

Currently there are two ongoing clinical trials undergoing recruitment in the United States and worldwide to evaluate investigational medicines that could possibly treat TTR.

There are several distinct urticarial syndromes including:

- Muckle–Wells syndrome

- Familial Mediterranean fever

- Systemic capillary leak syndrome

Until recently, doctors have diagnosed patients with FHS based on clinical observations and how well they fit the disease description, usually occurring in early childhood. Molecular genetic testing is also used now to test for genetic mutations. By performing a sequence analysis test of select exons, mutations can be detected in exon 34 of the SRCAP gene. This mutation has been observed in 19 patients to date.

In most cases, if the patient shows classic facial features of FHS, the molecular testing will show a mutation on the SRCAP gene.

A team from the National Institute of Nutrition, Hyderabad conducted research in the affected regions and found out that the disease was confined to a dalit colony. They also found out that the disease was affecting people of all age groups and it was non-contagious and non-infectious. This research was further followed by research conducted over four years (1984–88) by the Indian Council of Medical Research (ICMR). Study groups involving experts from different medical fields were formed and these groups visited the affected places to conduct experiments. They were also helped by the Health Department of Karnataka who also provided a team to assist them. Despite undertaking extensive research, the team could not come into any conclusion on the cause of the disease. The team wound up in 1988 and this was the first phase of their research. The second phase of the research by ICMR was started in the year 2001 under the leadership of S. S. Agarwal of the Sanjay Gandhi Post-graduate centre in Lucknow. Some of the activities in the second phase were to x-ray the patients and also study their family history to see if the disease was related to genetics. The study reported that Handigodu syndrome is a syndrome of familial spondyloepi(meta)physeal dysplasia. It is inherited as an autosomal dominant trait. All the presentations of the varied manifestation of the disease could be explained as being caused by defective development of bones as a result of monogenic disorder.

FHS shares some common features with Rubinstein–Taybi (due to overlapping effects of mutations on SRCAP), however cranial and hand anomalies are distinctive: broad thumbs, narrow palate, and microcephaly are absent in Floating-Harbor Syndrome. One child in the UK has a diagnosis of microcephaly alongside Floating–Harbor syndrome.

Symptoms of the disease are an acute pain and swelling in the hips and knee joints. Some of the other characteristics of this disease are dwarfism from birth, deformation of the limbs after age seven and death as early as between 25 and 30 years or even younger. Depending on the mobility of the affected patients, the disease has been identified with three severities: in mild to moderate cases, the patient is able to walk with difficulty, in severe cases mobility is very restricted, whereas in acute cases the limbs are bent and badly crippled making the patients crawl.

Multiple sclerosis (neurologic pain interpreted as muscular), Myalgic Encephalomyelitis (chronic fatigue syndrome), Myositis, Mixed connective tissue disease, Lupus erythematosus, Fibromyalgia syndrome, Familial Mediterranean fever, Polyarteritis nodosa, Devic's disease, Morphea, Sarcoidosis

Office of Rare Diseases listed Lyngstadaas syndrome as a "rare disease". This means that Lyngstadaas syndrome, or a subtype of Lyngstadaas syndrome, affects less than 200,000 people in the US population.

Orphanet, a consortium of European partners, currently defines a condition rare when if affects 1 person per 2,000. They list Lyngstadaas syndrome as a "rare disease".