The test for negligence is straightforward and summarised in the frequently cited judgment of Kruger v Coetzee 1966 2 SA 428 A at 430 E-H 

If a reasonable person would have foreseen the reasonable possibility of harm and would have taken reasonable steps to prevent it happening, and the person in question did not do so, negligence is established.

It is the facts of each case which may complicate the application of the principle.

This recent judgment is a reminder that the approach in any case is no more than a specific application of the generally expressed test for negligence.

In the case of a medical malpractice claim, a medical practitioner diagnosing and treating a patient is expected to exercise the level of skill, care and diligence exercised at the time by members of the profession to which they belong.  A deviation from that standard which causes harm results in culpability.

The same principles apply where the claim is for negligence against any expert in their field.

What level of skill, care and diligence may be expected in the particular context is often the subject of extensive and much debated expert evidence and legal argument.