In 2010 the Ghent nosology was revised, and new diagnostic criteria superseded the previous agreement made in 1996. The seven new criteria can lead to a diagnosis:

In the absence of a family history of MFS:

1. Aortic root Z-score ≥ 2 AND ectopia lentis

2. Aortic root Z-score ≥ 2 AND an FBN1 mutation

3. Aortic root Z-score ≥ 2 AND a systemic score* > 7 points

4. Ectopia lentis AND an FBN1 mutation with known aortic pathology

In the presence of a family history of MFS (as defined above):

1. Ectopia lentis

2. Systemic score* ≥ 7

3. Aortic root Z-score ≥ 2

- Points for systemic score:

- Wrist AND thumb sign = 3 (wrist OR thumb sign = 1)

- Pectus carinatum deformity = 2 (pectus excavatum or chest asymmetry = 1)

- Hindfoot deformity = 2 (plain pes planus = 1)

- Dural ectasia = 2

- Protrusio acetabuli = 2

- pneumothorax = 2

- Reduced upper segment/lower segment ratio AND increased arm/height AND no severe scoliosis = 1

- Scoliosis or thoracolumbar kyphosis = 1

- Reduced elbow extension = 1

- Facial features (3/5) = 1 (dolichocephaly, enophthalmos, downslanting palpebral fissures, malar hypoplasia, retrognathia)

- Skin striae (stretch marks) = 1

- Myopia > 3 diopters = 1

- Mitral valve prolapse 1⁄4 1

The thumb sign (Steinberg's sign) is elicited by asking the patient to flex the thumb as far as possible and then close the fingers over it. A positive thumb sign is where the entire distal phalanx is visible beyond the ulnar border of the hand, caused by a combination of hypermobility of the thumb as well as a thumb which is longer than usual.

The wrist sign (Walker's sign) is elicited by asking the patient to curl the thumb and fingers of one hand around the other wrist. A positive wrist sign is where the little finger and the thumb overlap, caused by a combination of thin wrists and long fingers.

During pregnancy, even in the absence of preconception cardiovascular abnormality, women with Marfan syndrome are at significant risk of aortic dissection, which is often fatal even when rapidly treated. Women with Marfan syndrome, then, should receive a thorough assessment prior to conception, and echocardiography should be performed every six to 10 weeks during pregnancy, to assess the aortic root diameter. For most women, safe vaginal delivery is possible.

Marfan syndrome is expressed dominantly. This means a child with one parent a bearer of the gene has a 50% probability of getting the syndrome. In 1996, the first preimplantation genetic testing (PGT) therapy for Marfan was conducted; in essence PGT means conducting a genetic test on early-stage IVF embryo cells and discarding those embryos affected by the Marfan mutation.

Radiological appearances include:

- Pseudofractures, also called Looser's zones.

- Protrusio acetabuli, a hip joint disorder

Biochemical features are similar to those of rickets. The major factor is an abnormally low vitamin D concentration in blood serum.Major typical biochemical findings include:

- Low serum and urinary calcium

- Low serum phosphate, except in cases of renal osteodystrophy

- Elevated serum alkaline phosphatase (due to an increase in compensatory osteoblast activity)

- Elevated parathyroid hormone (due to low calcium)

Furthermore, a technetium bone scan will show increased activity (also due to increased osteoblasts).