Diet is a good point - hers is pretty bad but a blood test is a problem as she has a massive needle phobia!! rockinhippy do you think she should have supplements anyway?
Some supplements might be a good idea as several vitamin deficiencies have depression as a symptom.
But with B12 deficiency you need to be sure that there isn't any underlying absorption problem, if there is, then supplements containing B12 won't help, but will skew any future blood test results.
I learnt that one the hard way with my own DD, we have a very strong family history of autoimmune Pernicious Anaemia. DDs blood results were on the low side, but not low enough for our doctor to take seriously & treat, as she had been taking supplements, but it's the wrong form of B12, so doesn't help her health problems. If you can afford it though & there are reasons to suspect an absorption problem of some sort, then there is a private uMMA urine test which is the gold standard test. Unfortunately NHS tests are known to be unreliable, but not all doctors work on that basis.
IF you are sure there are no underlying problems such as family members with PA, she's had ongoing stomach trouble, chrohns, colitis, IBS etc, certain drugs such as nitrous oxide for teeth extraction etc which block B12 absorption, possible parasite such as a tape worm & she doesn't eat a lot of animal produce, - (sorry, I've been on a huge learning curve with this stuff lately & it's been a real eye opener for us health wise, so I can ramble on
). - Then yes, she could well benefit from a Complete B vitamin supplement & maybe an added B12 sublingual spray or lozenge too to boost her B12 right up - it's a water soluble vitamin, so she can't have too much. She will also need to take Folic acid to help the body absorb the B12.
Other B Vit deficiencies, such as B6 (I think) also have Depression as a symptom
& Vitamin D is something most of the U.K. is deficient in, especially in winter, this causes depression too(& other symptoms) the NHS good levels for this are actually pretty low, so where as you can over dose on it, you can safely take quite a lot -
my DDs vit D was low, so I gave her a "loading dose of 10,000iu of D3, plus 200pu of K2, with a spoonful of coconut oil (fat to help absorb it) daily for 2 weeks & her next blood test it was a bit high (as far as NHS goes) & I now give her 5000iu, plus Vit K 2/3 times a week as a maintenance dose, she also takes magnesium & 5htp too, though I've heard good things about sAME too for helping depression, so I'm looking into swapping 5htp for that
My own DD does have a lot more going on than the depression & anxiety though, pain, insomnia etc, but she actually chose to meet 2 of her school friends in town on her own this week & insisted that she would be okay with the wheelchair on her own, she's also,looking forward to visiting school this week, this might not sound a lot, but it is a HUGE step forward for her, so I do think the supplements help