A diagnosis, an impairment and a functional impact are three different things

These three words get used as though they were interchangeable. They are not, and the difference between them explains one of the most expensive mistakes I see people make.

The mistake looks like this. A decision goes against you. You conclude that the problem must be the diagnosis: that it is too vague, or contested, or that a more serious label would have carried more weight. So you spend two years and several thousand dollars pursuing a new opinion. You get one. Nothing changes.

Nothing changes because diagnosis was never the thing being decided.

The three layers

Diagnosis is the clinical name for what is happening. It identifies the condition. On its own it establishes nothing about capacity, and it is not one of the tests.

Impairment is a loss of, or damage to, a body function or structure. This is what the Act actually asks about. It has to be attributable to a physical, neurological, sensory, cognitive, intellectual or psychosocial cause.

Functional impact is what the impairment prevents or limits in daily life, assessed across six named activities: communication, social interaction, learning, mobility, self-care and self-management.

The chain runs diagnosis to impairment to functional impact, and the decision is made almost entirely at the third link. A file weighted heavily toward the first has documented the least useful of the three.

A note on the two different sets of classifications

Worth pausing here, because this trips up almost everyone including clinicians.

Two separate classifications are at work. Each has six entries, and they are not the same list.

Impairment categories: intellectual, cognitive, sensory, neurological, physical, psychosocial. These describe what kind of function has been lost.

Activity domains: communication, social interaction, learning, mobility, self-care, self-management. These describe where the loss shows up in your life.

An impairment sits in one of the categories. Its consequences land in one or more of the domains. So when a report says someone was "assessed across the six domains", it is worth checking which list is meant, because a file can be thorough about one and silent on the other.

Same diagnosis, two different cases

This is where it becomes concrete. What follows are two patterns I have read many times over. They are composites rather than real files, and no individual's case is described here.

Take two people with chronic low back pain. Similar duration, similar imaging, the same three words at the top of the referral letter.

The first person. The impairment is physical: reduced range of movement through the lumbar spine, reduced physical endurance, and pain that limits how long any position can be held. The functional impact lands in mobility and self-care. She cannot walk further than about eighty metres, cannot sit through a meal, needs seated support to shower and cannot manage lower body dressing without help.

The evidence that establishes her case is walking tolerance, sitting and standing tolerance, transfers, and the tasks in her bathroom and bedroom.

The second person. Same diagnosis. His back is not the limitation. Years of disrupted sleep have degraded attention and working memory; sustained pain has consumed the cognitive capacity that planning requires; his mood regulation has deteriorated. The impairments here are cognitive and psychosocial, not physical. The functional impact lands in self-management and learning. He cannot reliably track which medication was taken when, misses appointments he intended to attend, cannot follow a sequence of instructions through to the end, and has handed the household administration to his partner without either of them ever calling it a support need.

The evidence that establishes his case has almost nothing to do with his spine.

Now the problem. Both men and women in this position are typically sent for the same assessment, because the diagnosis is the same. The second person accumulates walking tolerance measurements and sitting tolerance measurements, all of them competent, none of them addressed to the thing that has actually reduced his capacity. His file grows and his case does not.

Different diagnoses, the same case

The symmetry runs the other way too, and it is just as instructive.

Consider someone with fibromyalgia and someone with long-standing inflammatory arthritis. Different conditions, different mechanisms, different specialists, different treatment.

Both may present with the same impairments: reduced physical endurance, and cognitive fatigue severe enough to interfere with sustained attention. Both may land in the same domains, with the same consequences for work, household management and social participation. The evidence required to establish either case can be near-identical.

Put the two observations together and the conclusion is unavoidable. The same diagnosis can require completely different cases. Different diagnoses can require the same case. Therefore diagnosis is not the variable that determines what has to be established.

Why the category matters for your notice

There is a practical reason to know which impairment category your limitation belongs to.

Since 1 January 2025, people who meet access receive an impairment notice, which lists the impairments they were granted access for, not their diagnoses. Existing participants are being issued notices progressively, so if you have been in the scheme for some years you may not have seen one yet.

Once you are in the scheme, a support has to address needs arising from an impairment on that notice. Which means a pain-related impairment that was never recorded can leave supports for it refused, even though you are already a participant, and even though nobody disputes the condition.

That is not the end of it. In Forrest and NDIA [2025] ARTA 1131 the Tribunal read the provision in the present tense (the impairments that count are the ones the requirements are met for now, not only the ones recorded years earlier) and applied the Note to s 34(1), which allows an unrecorded impairment to matter through its impact on a recorded one. Pain that did not itself meet the disability requirements was aggravating accepted psychosocial impairments, and the supports directed at controlling it were funded. That argument still has to be made, and the mechanism still has to be documented.

The second person above is exposed here. If his access was granted on a physical impairment, because that is what the file emphasised, then his cognitive and psychosocial impairments may not appear on the notice at all. The supports he most needs are attached to impairments the record never established.

Notices are reviewable, and a variation can be sought. But you cannot ask for a category to be added if you have never worked out which category your limitation belongs to.

What to do with this

If a decision has gone against you, the useful question is not whether your diagnosis is strong enough. It is which impairment you were relying on, which category it sits in, and which of the six activity domains the file establishes it affects.

Most often the answer is that the diagnosis was never in doubt, the impairment was recorded loosely or not at all, and the functional impact was left to be inferred. Decision-makers are not permitted to infer it.

A better diagnosis will not fix that. Establishing the middle and third layers will.

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Where this leaves your file

If your diagnosis is well established and the decision still says insufficient information, the problem is translation. The material is likely to be there, described in clinical terms rather than in terms of impairment and named activity. This is the most common finding I make.

If your condition is documented but the impairment behind it was never named, the gap is in functional evidence. What is missing can be assessed and documented.

If you recognised the second person in this article, it is worth checking what your file and your impairment notice actually say. The limitation that matters most to you may be the one nobody wrote down.

An Evidence Audit identifies which of these applies to you, in writing, with the reasoning set out. If you would rather work out whether that fits your situation by talking it through first, the introductory call is free.

Observations drawn from published Tribunal and Court reasons, current as at the compilations cited. Nothing here is legal advice, and nothing here predicts an outcome. Every file turns on its own facts.