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Free Androgen Index
(a.k.a FAI)
minimum sample volume required ~ 5ml
Tube type: SST
Other acceptable tube types
Lithium heparin (pale green)
Reference range
≥18 years old: 0.8 – 9
Units
%
Linked to
Testosterone, SHBG
Turnaround Time
1
day
Department: Biochemistry
Clinical Application
Free androgen index is a calculated test using testosterone and SHBG.
FAI cannot be calculated where testosterone is <0.4nmol/L.
FAI can be used in the investigation of polycystic ovarian syndrome (PCOS). If symptoms correlate, an FAI >9% supports the diagnosis of PCOS, but is not diagnostic.
As per Bath and North East Somerset, Swindon and Wiltshire (BSW) guidelines, alongside total testosterone, FAI may be used to determine whether testosterone replacement is appropriate in those with low sexual desire which is associated with menopause. It may also be used, alongside testosterone, to monitor testosterone hormone replacement therapy.
Webpage created: September 2025.
