Misdiagnosis & Delayed Diagnosis Lawyers in Ontario
The right diagnosis, made on time, changes everything. When a serious condition is missed, mistaken for something else, or caught too late, the window for effective treatment can close — and the consequences can last a lifetime. Diamond and Diamond helps Ontario patients and families find out what went wrong, and what the law can do about it.
When the right diagnosis comes too late
A diagnosis is the starting point for everything else in medicine. Get it wrong — or get it late — and every decision that follows is built on the wrong foundation.
Diagnostic error is one of the most common issues in Canadian medico-legal cases. In a review of its closed primary care files from 2016 to 2020, the Canadian Medical Protective Association identified diagnostic error in 1,260 cases involving family physicians alone. Behind each of those files is a person whose cancer, infection, heart condition, or neurological event was not recognized when it should have been.
Not every wrong or delayed diagnosis is negligence. Medicine involves judgment, and some conditions are genuinely hard to detect. The legal question is different: did the care fall below the standard a reasonable practitioner would have met in the same circumstances — and did that failure change the outcome?
The team of lawyers at Diamond and Diamond have experience handling misdiagnosis and delayed diagnosis claims across Ontario. A free consultation is the first step toward understanding whether what happened to you or your family member may support a claim.
Misdiagnosis, delayed diagnosis, and missed diagnosis
The law treats these as variations of the same core failure: a condition that a reasonable practitioner would have identified and acted on, but that went unrecognized. The difference lies in how the failure unfolds.
Wrong diagnosis
The patient is told they have one condition when they actually have another. Treatment targets the wrong problem while the real one advances untreated — and the treatment itself may cause harm.
Delayed diagnosis
The correct diagnosis is eventually made — but only after a delay that allowed the condition to progress. In cancer, stroke, and serious infections, that delay can be the difference between full recovery and permanent harm.
Missed diagnosis
The condition is never identified at all. The patient is told nothing is wrong, or the symptoms are attributed to something minor, and they are discharged without the investigations their presentation called for.
The lost window
Almost every misdiagnosis and delayed diagnosis claim in Ontario comes down to a window of time: the period between when the condition should reasonably have been identified and when it actually was. What happened inside that window — and what a timely diagnosis would have changed — is what a claim must establish.
Warning signs present
Symptoms are reported, test results come back, or a presentation appears that a reasonable practitioner would investigate.
The condition advances
Investigations are not ordered, results are not followed up, or symptoms are attributed to something minor. Treatment that could have changed the outcome is not given.
Correct diagnosis made
The true condition is finally identified — often at a more advanced stage, with fewer treatment options and a worse outlook.
Reconstructing that window requires the complete medical record: clinic notes, referral letters, imaging, laboratory results, and emergency department charts. That is why requesting your records early matters so much.
Conditions commonly involved in diagnostic error claims
Any condition can be misdiagnosed, but claims tend to involve conditions where time directly determines the outcome. In its review of emergency medicine files, the CMPA found stroke, ischemic heart disease, and infections of the respiratory and digestive systems among the conditions most often involved in diagnostic cases.
Cancer
Delayed workup of breast, colorectal, lung, prostate, and skin cancers — including imaging findings and screening results that were never actioned.
Heart attack & cardiac events
Chest pain attributed to indigestion, anxiety, or muscle strain; abnormal ECGs or cardiac markers not acted on.
Stroke & TIA
Early neurological signs dismissed, or transient symptoms discharged without the urgent assessment they required.
Sepsis & serious infections
Deteriorating vital signs and infection markers that were not recognized or escalated in time.
Meningitis
Fever, headache, and neck stiffness treated as a routine viral illness — particularly dangerous in children.
Appendicitis
Abdominal pain discharged as gastroenteritis or constipation, followed by rupture and serious complications.
Pulmonary embolism & DVT
Leg pain, breathlessness, or low oxygen readings not investigated for a clot before it became life-threatening.
Pregnancy complications
Ectopic pregnancy, pre-eclampsia, and other obstetric emergencies where warning signs were not investigated promptly.
Where diagnostic errors happen
Diagnosis is a chain that runs through many hands — family physicians, emergency departments, radiologists, laboratories, and consultants. A claim may involve any link in that chain, and often more than one.
In hospitals & emergency departments
- Triage that understated the urgency of a presentation
- Patients discharged without the imaging or blood work their symptoms called for
- Abnormal results returned after discharge with no follow-up
- Imaging misread, or radiology findings never communicated to the treating team
- Information lost between shifts, wards, and departments
A pattern worth noting
Patients who return to the emergency department with the same worsening complaint — and are sent home again — appear repeatedly in diagnostic error reviews. A return visit is a signal to re-examine the working diagnosis, not to repeat it.
In family practice & community care
- Symptoms attributed to a known condition, age, or stress without investigation
- Screening results or referrals that were ordered but never completed or tracked
- Test results filed without review or without informing the patient
- Delayed referral to the right consultant as symptoms progressed
- Walk-in and virtual visits where no one carried the file forward
A pattern worth noting
Diagnostic error is a major issue in primary care: of 6,175 closed CMPA cases involving family physicians between 2016 and 2020, independent reviewers identified diagnostic error in 1,260. Follow-up failures — results and referrals that fell through the cracks — are a recurring theme.
“If you suspect a diagnosis was missed or delayed, request complete copies of your medical records right away — including test results, imaging reports, and referral letters. The record is where the timeline of your care lives.”— Diamond & Diamond Lawyers
When does a wrong or late diagnosis become negligence?
The law does not demand perfection from health care providers. It demands the level of care a reasonable practitioner in the same field would have provided in the same circumstances. Claims typically focus on one or more of these recurring patterns.
Inadequate history or examination
Key symptoms were never asked about, or a physical examination that the presentation called for was not performed.
Failure to order indicated tests
Imaging, blood work, or other investigations that a reasonable practitioner would have ordered were not requested.
Misread results or imaging
Test results or scans were interpreted incorrectly, and findings that pointed to the true condition were overlooked.
Results not followed up
Abnormal results came back but were never reviewed, never communicated to the patient, or never acted on.
Premature closure
The first plausible explanation was accepted and alternatives were never considered — even as symptoms persisted or worsened.
Failure to refer or escalate
The patient was not sent to the right consultant, or a deteriorating patient was not escalated, in the time frame their condition required.
Whether any of these amounts to negligence in a specific case depends on the medical record and on opinions from qualified medical professionals about what the standard of care required. That analysis is the foundation of every claim — and it is not something families can access on their own, which is why an early conversation with a lawyer matters.
Speak to a lawyerProving a misdiagnosis or delayed diagnosis claim in Ontario
Every medical negligence claim in Ontario rests on the same three pillars. Diagnostic error cases live or die on the second one.
1. Breach of the standard of care
The care fell below what a reasonably competent practitioner in the same field would have provided in the same circumstances. Establishing the standard — and the departure from it — relies on the medical record and on opinion evidence from qualified medical professionals.
2. Causation
The delay must have made the difference. Ontario courts require proof, on a balance of probabilities, that a timely diagnosis and treatment would likely have avoided the unfavourable outcome — the principle confirmed by the Court of Appeal in Cottrelle v. Gerrard, 2003 CanLII 50091 (ON CA).
Showing that earlier treatment would merely have given the patient a better chance is not enough on its own. This is often the hardest part of a diagnostic error case, and it is where careful analysis of the lost window becomes decisive.
3. Damages
The breach caused real, compensable losses — a worse medical outcome, additional treatment, lost income, ongoing care needs, and pain and suffering. If the outcome would have been the same with a timely diagnosis, there is no claim, no matter how clear the error.
What a successful claim may cover
Damages in an Ontario medical negligence action are designed to restore, as far as money can, what the negligence took away. In a diagnostic error case, that means the difference between the outcome you got and the outcome a timely diagnosis would likely have delivered.
Past medical & out-of-pocket costs
Treatment, medications, travel for care, and other costs already paid because the condition advanced further than it should have.
Future care costs
Ongoing treatment, rehabilitation, therapy, medications, equipment, and home modifications projected over a lifetime where needed.
Income loss & earning capacity
Wages lost to date and the reduction in future earning capacity caused by a worse medical outcome.
Pain & suffering
Non-financial damages reflecting physical pain, emotional distress, and loss of enjoyment of life, assessed against previous case law.
Housekeeping & home maintenance
The value of household work you can no longer perform and must replace or forgo because of the injury.
Family Law Act claims
In Ontario, close family members may claim for their loss of care, guidance, and companionship, and for expenses they incurred for the injured person’s benefit.
Suspect a diagnosis was missed or delayed?
Speak directly with our legal team. Free consultations available 24/7.
1-800-567-HURTServing patients and families across Ontario
Diamond and Diamond handles misdiagnosis and delayed diagnosis claims arising from hospitals, emergency departments, clinics, and community care across the province.
Contacting the lawyers at Diamond and Diamond after my accident was the best decision I could have made. Within a few hours of contacting them, I was set up with an occupational therapist, social worker and PSW. It made returning home from the hospital much more bearable. Rachel Cleland and Nastassia Ivanova have been incredibly supportive. Always willing to answer any of my questions or concerns in a timely manner. I highly recommend the team at Diamond and Diamond lawyers.— Rhonda
Jul 14, 2026
Frequently asked questions about misdiagnosis claims
When should I contact a lawyer about a misdiagnosis or delayed diagnosis?
Is every wrong or late diagnosis medical malpractice?
What do I need to prove in a misdiagnosis claim?
What if my condition would have been serious even with a timely diagnosis?
Can I claim against a hospital, walk-in clinic, or laboratory?
How are legal fees handled?
What records and information should I keep?
Looking for more answers? See all medical malpractice FAQs →
This page provides general information about Ontario law and is not legal advice. Every situation is different. For advice about your circumstances, speak with a lawyer. Statistics cited reflect publicly reported figures from the Canadian Medical Protective Association for the periods indicated. Past results are not necessarily indicative of future results, and the amount recovered and other litigation outcomes will vary according to the facts in individual cases.
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