Diagnosis: Differential diagnosis

We essentially use a differential diagnosis as a tool to help us escape a potential tunnel vision. The official meaning of a differential diagnosis (diacrisis) is:

Differential diagnosis is a scientific method of making a diagnosis from a list of possible conditions that a particular patient may suffer from, given the signs and symptoms known at that time.

Don't take this too literally, because that would mean, for example, diagnosing "skin worms" simply because the fish are rubbing (flaring). Since there are many conditions that can cause a fish to rub/flare, you're just guessing. It's more a spectrum of symptoms that could point in a certain direction. The actual examination must demonstrate that the diagnosis matches the assumption of what the problem might be.

In essence, we're turning it around. We conduct our investigation and, if we're lucky, make a diagnosis. In other words, we find something that might be detrimental to the fish's well-being. We then apply the definition of differential diagnosis above by examining whether the diagnosis aligns with the fish's abnormal behavior (the disease).

This all sounds very complicated, but it isn't. I'll try to illustrate this with an example.

Example

One fateful day, Ron discovers his fish lying lifeless and with their fins pinched at the bottom of the pond. He decides to investigate ( see the article "Diagnosis" ). During the microscopic examination, he detects the presence of skin worms on his fish. While checking the water quality, he noticed that their ammonia levels were far too high and then realized that he had overfed them far too quickly this spring, preventing the filter from reaching full capacity. He takes measures to lower the ammonia and decides to use a worming agent to treat the skin worms. A week later, the fish have not recovered; some have already died. Ron calls in an expert.

The expert redoes the entire examination. He knows about the ammonia spike from the anamnesis. This problem has been resolved, however, because he no longer measures ammonia. However, during the microscopic examination, he finds large amounts of Costia. The skin worms Ron mentioned are nowhere to be found.

If Ron had used the differential diagnosis, he would have found that lying on the bottom with pinched fins isn't a common symptom of a skin fluke outbreak. He should have investigated further and perhaps improved his microscope skills with a course, because he apparently overlooked the Costia.

The moral of the story is that Ron was happy to have found something and acted accordingly. However, the symptoms didn't match his discovery. The diagnosis was therefore incomplete, because besides skin worms, the fish were mainly suffering from Costia (a small parasite that is often overlooked).

Author: Joop van Tol (www.koitoday.nl)