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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)
Special instructions
Please note: The combined oral contraceptive pill (COCP) should be stopped for at least 3 months prior to biochemical assessment for ?PMOS (as per NICE CKS).
Reference range
Females: <14 years of age: No range, 14 - 17 years of age: 0.6 - 6.5, 18 - 46 years of age: 0.7 - 10.9, 47+ years of age: 0.2 - 6.8
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 Polyendocrine Metabolic Ovarian Syndrome (PMOS). If symptoms correlate, an FAI above the age-related reference range supports the diagnosis of PMOS, 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 updated: July 2026.
