Brain fog blood tests in Australia

Healthdirect: "Brain fog is not a medical term, but it's how most people who experience memory or cognition changes refer to these conditions." Jennings et al. 2022: it is not a recognised medical diagnosis. Ross et al. 2025 reviewed its use across more than a dozen chronic diseases and found it used as a distinct symptom, a syndrome, or an inherently ambiguous term. Those are three different claims. This page treats it as the third: a description that still deserves a careful filter.

Low iron, B12 deficiency, an underactive thyroid, high or swinging glucose, perimenopause, sleep debt, depression, and post-viral illness can all be reported as fog. They do not share a pathway. A panel that "diagnoses brain fog" is selling a label the colleges do not use.

The RACGP blood list below is the set used when a GP is excluding medical causes of cognitive change, including in older adults. A few foggy days after a bad week of sleep is not that presentation. Red flags that skip the wait-and-see step include sudden confusion, one-sided weakness, trouble speaking, a new severe headache, or a progressive change that is interfering with work or safety. Those belong with a GP or emergency care the same day, not with a preventative membership page.

In g/L. WHO and Australian practice treat anaemia as under 120 g/L in non-pregnant women and under 130 g/L in men. A normal haemoglobin does not rule out low iron stores or a B12 problem.

Mean cell volume, in fL. Small cells (typically under 80) point toward iron deficiency or thalassaemia. Large cells (typically over 100) point toward B12 or folate deficiency, alcohol, or liver disease. Mild deficiency can still have a normal MCV.

In µg/L. Australian labs commonly flag under 30 as low. A result between 15 and 30 can read as "not deficient" against the WHO well-population cut-off and "depleted" against Australian practice.

Total B12 in pmol/L. Current Australian Prescriber bands: below 133 likely deficient, 133 to 258 indeterminate, above 258 unlikely deficient. Active B12 (holotranscobalamin) is the cell-available fraction. Do not treat the two numbers as interchangeable, and do not convert a US pg/mL page without the 0.738 factor.

In mIU/L. The RCPA manual default adult interval is 0.4 to 4.0, and individual labs vary. Always use the interval printed on your report. A wellness page that calls 0.5 to 2.0 "optimal for cognition" is not Australian college guidance.

In pmol/L. These measure unbound thyroid hormone. They are not first-line on their own. RCPA advice is to add free T4 when TSH is abnormal, and free T3 when TSH is suppressed but free T4 is still in range.

Fasting glucose in mmol/L. HbA1c in mmol/mol, often with a percentage beside it. They are the same glycation result in two units, not two tests. Diabetes is 48 mmol/mol (6.5%) or above; prediabetes is 42 to 47 mmol/mol (6.0% to 6.4%).

Folate in nmol/L. Calcium in mmol/L, often as corrected calcium. Both are on the RACGP cognitive list. A low folate can travel with a B12 problem; a calcium outside the printed interval is a same-week conversation, not a brain-fog footnote.

Vitamin B12 is a cofactor for methionine synthase, which feeds DNA synthesis and myelin through the folate cycle, and for methylmalonyl-CoA mutase, which handles odd-chain fatty acids. When the second enzyme stalls, methylmalonic acid rises. The result can be megaloblastic anaemia, demyelination, or both. That pathway is settled. A normal FBE does not close the question if the history still points here.

Australian Prescriber (2026) lists memory impairment, irritability, depression, delirium, and psychosis among psychiatric manifestations, and says neurological or psychiatric disease can be present without haematological abnormalities. It is reasonable to exclude B12 deficiency in older adults being investigated for cognitive decline. That is a screen, not a claim that a low-normal B12 is why you lost a word this week.

Current Australian guidance reads total B12 as below 133 pmol/L (likely deficient), 133 to 258 pmol/L (indeterminate), and above 258 pmol/L (unlikely deficient). Active B12 is the usual follow-up in the grey zone: below 25, 25 to 70, and above 70 pmol/L. A US page that talks in pg/mL is using a different unit.

Australian Prescriber lists vegan or vegetarian diets, older age, metformin, PPIs or H2-blockers used for more than two years, bariatric or ileal disease, pernicious anaemia, and nitrous oxide. In Australians over 50, deficiency prevalence is cited at 5.2% to 6.3%. A whole-population adult rate has not been published in a form worth quoting. Item 66838 funds total B12 and/or active B12 once per 11 months in most people; the item description was updated on 1 July 2025 to add active B12 alongside total B12.

When blood glucose stays high, the kidneys spill glucose into urine and water follows it. That osmotic diuresis causes thirst, dehydration, nocturia, and broken sleep. The fog can be the sleep as much as the sugar. When glucose falls, the brain is short of fuel and attention goes first. Both are standard pathophysiology, clearest once diabetes or hypoglycaemia is established.

Muscle cells take up glucose through GLUT4 transporters that insulin has to recruit. In insulin resistance the pancreas compensates by making more insulin, so fasting glucose and HbA1c can sit inside the reference interval for years while fasting insulin is already high. That compensation pattern is settled biochemistry, and Bell et al. 2020 build the Australian screening case on it. It is not, on its own, proof that a high fasting insulin is why you feel foggy; it is a reason not to stop at a normal HbA1c if the metabolic history still looks off.

The RACGP cognitive list uses glucose, not HbA1c, as the first-line sugar test. HbA1c estimates average glucose over roughly two to three months and does not need fasting. Diabetes is diagnosed at HbA1c 48 mmol/mol (6.5%) or above, confirmed on a second test; prediabetes is 42 to 47 mmol/mol (6.0% to 6.4%), per the Australian Diabetes Society. Fasting insulin is not a first-line brain-fog test.

The ABS National Health Measures Survey 2022 to 2024 found about one in fifteen Australian adults had diabetes: 6.6% by the fasting-glucose algorithm and 6.4% by HbA1c. Use that range. A single headcount is not printed on the ABS release.

Broken sleep and obstructive sleep apnoea both present as fog, and blood cannot show either. The Busselton Healthy Ageing Study found moderate-to-severe OSA in 20.2% of men and 10.0% of women aged about 46 to 69. Doctor-diagnosed rates are much lower, which is the gap, not a contradiction. A sleep history, and sometimes a sleep study, can close this. A ferritin cannot.

Cognitive complaints travel with mood. There is no AIHW figure that cleanly answers "what share of foggy Australians have depression," and this page will not invent one. A normal blood report does not make the fog imaginary, and it does not replace a mental-health conversation.

Jennings et al. 2022 studied brain fog as a lay term for cognitive dysfunction after COVID-19, not as a diagnosis. Published Australian long-COVID figures are a range, not a single national rate: an AIHW 2022 review put then-available estimates at 5% to 10% of COVID-19 cases with symptoms beyond 12 weeks. Blood can exclude some other causes. It cannot confirm this one.

RACGP cognitive-assessment guidance asks about medicines, especially anticholinergic and psychotropic agents, which can dull thinking in older adults. Australian fatigue guidelines ask the same about complementary products, alcohol, and sleep. They do not publish a named drug list, and this page will not invent one. Bring the box, the bottle, and an honest week of sleep to the appointment.