When we visit a doctor for diagnosis and treatment, we expect to receive proper care in accordance with professional medical standards. However, in some cases, patients may receive an incorrect diagnosis, leading to inappropriate treatment, worsening of the condition, or, in severe cases, even death. Such incidents not only affect a person’s health but can also cause significant emotional and financial damage.

One common question is: Can a doctor be sued for a misdiagnosis? And if so, what legal grounds or evidence are required to file such a claim? The answers to these questions depend on the facts and legal considerations specific to each case.

Generally, a legal action against a doctor can be pursued under Thai civil or criminal law, or through a complaint filed with the relevant professional regulatory body. In the event of a patient’s death, the patient’s relatives also have the legal right under Thai law to initiate the proceedings.

Under civil law, the injured party may claim compensation under Section 420 of the Thai Civil and Commercial Code if the misdiagnosis was a result of the doctor’s negligence. Alternatively, the case may be filed as a consumer protection case, with the aim of seeking damages for substandard medical services.

In terms of criminal liability, if the doctor’s actions constitute gross negligence that results in the death of another person, criminal proceedings may be initiated under Section 291 of the Thai Penal Code, which carries a penalty of up to ten years’ imprisonment and/or a fine of up to 200,000 Baht. However, such criminal proceedings are relatively rare, as they require clear evidence that the misdiagnosis was a direct result of failing to adhere to professional standards, an issue that must be carefully examined from all legal and factual angles.

Additionally, the injured party may file a complaint with the Medical Council of Thailand if they believe the doctor failed to comply with medical standards or professional ethics. This may include cases where the doctor did not provide sufficient information before treatment or failed to respect the patient’s right to make informed decisions. Such actions may be considered violations under Section 32 of the Medical Profession Act B.E. 2525 (1982) and Section 8 of the National Health Act B.E. 2550 (2007).

This article aims to provide a clear understanding of the legal rights of patients, the available avenues for seeking accountability, and the key legal considerations in cases involving harm from medical misdiagnosis, so that you can protect your rights or those of your loved ones with confidence and accurate information.

What Is a Diagnostic Error?

A diagnostic error occurs when a doctor fails to correctly, completely, or promptly identify a patient’s medical condition. This also includes situations where diagnostic information is poorly communicated or insufficiently explained, leaving the patient unaware or misinformed about their own health status. This definition reflects the one provided in the policy “Patient Safety Goals: Simple Thailand 2018”Diagnostic errors generally fall into three main categories:

• Missed Diagnosis – This happens when a doctor fails to identify an existing medical condition, even though sufficient information was available to make the correct diagnosis.

• Wrong Diagnosis – This refers to diagnosing a patient with the incorrect illness, only for the correct condition to be discovered later. It often results in inappropriate or harmful treatment.

• Delayed Diagnosis – In this case, the correct diagnosis is eventually made, but only after an unreasonable delay. This can lead to delayed treatment, which may worsen the patient's outcome. For example, detecting cancer at a later stage.

Although the definition and characteristics of diagnostic errors are crucial for professional evaluation and risk prevention in the treatment process, they are generally not explicitly defined by law. Instead, they are typically outlined in internal regulations, guidelines, or protocols established by healthcare institutions or relevant public health agencies. These standards are intended to ensure a common understanding among medical professionals and serve as tools for assessment, monitoring, and preventing the recurrence of such errors in the future.

a) When a Misdiagnosis Is Not Considered Medical Negligence

Not all diagnostic errors are considered medical negligence under the law. In some cases, a doctor may act with full diligence and in accordance with accepted medical standards, yet still arrive at an incorrect diagnosis. These are instances where the error was unintentional and occurred despite appropriate professional care based on the available information at the time.

For example, a physician may choose a reasonable diagnostic or treatment method based on the patient’s symptoms, but the results may turn out to be inaccurate. In other cases, the condition itself may be unusually complex or present atypically, making accurate diagnosis genuinely difficult even when the doctor follows standard medical protocols.

An example of this would be a case where a doctor performs all necessary tests and monitors the patient according to standard procedures, but the illness presents in an uncommon way that makes it extremely hard to detect, despite the absence of negligence.

Notable Case Example: Supreme Court Judgment No. 5009/2562 (2019)

An illustrative case is Supreme Court Judgment No. 5009/2562, in which the plaintiff, Ms. A., a former nurse at the hospital operated by the first defendant (company), sought medical treatment for lower abdominal pain and vaginal bleeding. The second defendant, an obstetrician-gynecologist, diagnosed her with an ectopic pregnancy (การตั้งครรภ์นอกมดลูก) and recommended surgery to remove her ovary and uterus.

However, it was later discovered that the diagnosis was incorrect. In reality, the plaintiff had a normal intrauterine pregnancy (การตั้งครรภ์ในมดลูก) that ended in an incomplete miscarriage (การแท้งไม่สมบูรณ์), and the cyst identified during examination was actually a chocolate cyst, not indicative of an ectopic pregnancy. The plaintiff filed a lawsuit claiming damages on the grounds of medical malpractice arguing that the misdiagnosis led to an unnecessary surgical procedure.

The Supreme Court found that although the second defendant had indeed made an incorrect diagnosis, the condition was medically complex, and the plaintiff’s symptoms closely resembled those of an ectopic pregnancy. The court concluded that the second defendant had acted with reasonable care and had followed the accepted medical standards at the time. Therefore, the misdiagnosis did not constitute negligence.

Additionally, the plaintiff who held a Bachelor’s degree in Nursing and a Doctorate in Healthcare Business Administration had been fully informed of her condition and had consulted with her husband before consenting to the surgery. The court ruled that the surgical procedure was performed with the plaintiff’s informed consent, and thus, did not amount to medical malpractice or wrongful conduct under tort law.

b) When Misdiagnosis Constitutes Medical Negligence

On the other hand, a misdiagnosis may be considered medical negligence when it results from a lack of due care or failure to follow standard medical procedures. This includes situations where a physician neglects essential steps. For example, relying solely on a nurse’s report without conducting a direct physical examination, or ignoring critical medical information that should reasonably have been considered.

Such negligence can lead to an inappropriate course of treatment, causing harm to the patient. In previous rulings, the court has held that a doctor who makes a diagnosis and prescribes treatment based solely on secondhand reports without personally examining the patient, may be found negligent under the law.

Notable Case Example: Supreme Court Judgment No. 7452/2541 (1997)

This case involves medical malpractice arising from an incident in which the first defendant, a physician, diagnosed the plaintiff as having a stillbirth (ทารกเสียชีวิตตอนคลอด) and recommended a uterine curettage (การขูดมดลูก) to terminate the pregnancy. However, during the procedure, the doctor was under the influence of alcohol and used a surgical instrument in a manner that perforated the plaintiff's uterus, despite the uterus being structurally normal and not unusually thin or fragile.

Moreover, the instrument hooked onto the plaintiff’s small intestine, causing approximately five meters of intestine to prolapse through the vaginal canal. A subsequent medical team had to surgically remove part of the intestine to prevent infection. Further examination revealed no fetus in the uterus, and it could not be confirmed whether the plaintiff had actually been pregnant.

The Court ruled that the actions of the first defendant constituted negligence, as they failed to exercise the standard level of care expected of a medical professional, in accordance with Section 420 of the Thai Civil and Commercial Code. Expert testimony and medical evidence consistently indicated that the plaintiff’s uterus was normal and not in a condition that would have led to perforation absent negligent conduct. The incident involving the prolapsed intestine was clearly the result of the first defendant’s negligence.

Although the second defendant presented testimony from a medical expert not directly involved in the case in an attempt to challenge the plaintiff’s evidence, the Court found this testimony insufficient to outweigh the substantial and credible evidence presented by the plaintiff.

c) Supreme Court Judgment No. 5009/2562 (2019) VS Supreme Court Judgment No. 7452/2541 (1997)

Although the following two Supreme Court cases both involve instances of medical misdiagnosis, the rulings reached entirely different conclusions. This contrast highlights a key legal principle: not every misdiagnosis amounts to negligence or legal liability, particularly when a physician can demonstrate adherence to professional medical standards.

Key Takeaways:

• A misdiagnosis does not automatically amount to negligence.

• The physician acted reasonably and followed accepted medical standards given the complex symptoms.

• The patient provided informed consent, which supported the finding of no wrongful conduct.

• The ruling confirms that liability arises only when a doctor fails to meet the required standard of care, not simply because the diagnosis was incorrect.

Supreme Court Judgment No. 5009/2562

In this case, the second defendant, an obstetrician-gynecologist, made an incorrect diagnosis and initiated treatment before identifying the true cause of the patient’s symptoms. However, the Supreme Court ruled that the diagnosis was within the scope of accepted medical standards, and that the doctor had not acted negligently. The ruling was based on several factors:

• The condition was medically complex, and the symptoms closely resembled those of the initially diagnosed illness.

• The plaintiff (patient) had a nursing background and thus possessed basic health knowledge.

• The physician provided comprehensive information and treatment options to the patient before proceeding.

The Court concluded that the doctor had not acted carelessly or in violation of professional standards. Therefore, the misdiagnosis did not constitute medical malpractice.

Supreme Court Judgment No. 7452/2541

In contrast, this case involved a physician (the first defendant) who performed a medical procedure while clearly intoxicated, causing severe physical harm to the patient. Key facts include:

• A perforated uterus, despite there being no evidence that the uterus was abnormally thin or fragile.

• A section of the patient’s small intestine had to be surgically removed due to infection risk.

• Post-procedure investigations could not confirm whether the patient had ever actually been pregnant.

In this instance, the Court found that the physician exhibited gross negligence and failed to adhere to the standard of care. The doctor’s unfit condition for duty, coupled with the serious harm inflicted, clearly established liability under civil law.

Comparative Summary

While both cases involved misdiagnoses, the Court prioritized one critical question: Did the physician act with due care in accordance with professional standards?

If the physician made a misdiagnosis but acted honestly, diligently, and based on the information reasonably available at the time, without recklessness, the Court may rule that no legal wrongdoing occurred.

Conversely, if the circumstances show a lack of care, disregard for patient safety, or the physician’s unfitness to practice such as being under the influence the misdiagnosis, along with the resulting harm, may amount to legal negligence.

With this, Part 1 concludes. The second part of this article explores the legal steps patients may take, the burden of proof, and the critical elements involved in pursuing a misdiagnosis claim.