Can the Second to Fourth Digit Ratio (2D : 4D) Be a Marker to Determine Ankylosing Spondylitis Disease Activity?

The length ratio of the index finger (2D) to the ring finger (4D) (2D : 4D ratio) is considered a biomarker of prenatal sex hormone exposure. The 2D : 4D ratio is influenced by prenatal androgen and estrogen levels. Because ankylosing spondylitis (AS) influences men more frequently and severely than women, androgens are proposed to be related to AS pathogenesis. Estrogens have immune-modulating effects and reduce AS disease activity. The aim of this study was to assess the relationship between 2D : 4D ratio and AS disease activity.

In this study, 167 (43 female) patients diagnosed with AS were studied. The lengths of the second and fourth fingers were measured using a digital caliper. The 2D : 4D ratio was found by dividing the length of the second finger by the length of the fourth finger. AS disease activity was assessed with the Turkish version of the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). AS functional status was assessed with Bath Ankylosing Spondylitis Functional Index (BASFI). L-Schober, tragus to wall distance, finger to floor distance, and chest expansion were used to evaluate mobility.

In female patients, the right hand 2D : 4D ratios were higher than those in male patients. Biologic drug use was more frequent in males. The BASDAI scores were higher in female patients than in male patients. There were significant negative correlations between right and left hand 2D : 4D ratio and BASFI and BASDAI in female patients. There was no significant correlation between the 2D : 4D ratio and BASFI or BASDAI in male patients. We found a positive correlation between L-Schober and right hand 2D : 4D and a negative correlation between the left hand 2D : 4D ratio and finger to floor distance in female patients with AS.

The 2D : 4D ratio of the right and left hand was low in female patients with high BASFI and BASDAI and low spinal mobility (L-Schober) was also linked to low female 2D : 4D. The lack of strong associations between 2D : 4D and AS in male patients may have resulted from their higher use of biologics.