How does the Dentist Diagnose Decay?

We have seen how important it is to diagnose caries early, so how does the dentist do this? Well, first off they need to see you!

Sounds obvious I know, but if the reason you are going to see the dentist is because half your tooth has broken away or because you can’t sleep because of toothache– things have obviously progressed at lot further and will be more difficult and expensive to treat.

There are four main ways dentists’ diagnose decay:

Visually-looking at the teeth
(i) In order to check each of your teeth properly the tooth must be clean i.e. free of food and plaque and dry; this is why we squirt air on your teeth to remove the saliva and let us see everything nice and clearly.

(ii) Spotting decay can be obvious or not so obvious and sometimes it is only visible on X-rays.
Obvious decay forms a distinct hole. . So much decay has occurred that part of the tooth actually collapses to form a cavity.

Less obvious decay may be:

  • A white spot.This is the earliest form of decay and most often seen on the front surfaces of teeth near the gum margin. If the enamel around a pit or fissure (on the biting surface of the tooth) looks a dense, more opaque (solid) white colour, this normally suggests that caries is occurring underneath. A white spot on a wet tooth suggests that decay is over halfway through the enamel, possibly into dentine. If the white spot can only be seen after it is blasted with air, it suggests it is less than halfway through enamel and preventive treatment rather than filling may be attempted.
  • A brown spot.When a white spot roughens, it can pick up stain and turn a light brown colour.
  • A dark greyish shadow or translucent area on the marginal ridge in-between the back teeth, or the front teeth when the light is shone through them.

If these are left untreated they will eventually collapse and leave a large obvious hole, by which time the decay will have progressed close to the nerve.

Fluoride, whilst being our trusted ally in the fight against tooth decay causes a little difficulty when diagnosing decay, because it resurfaces (remineralises) the enamel on top. Because the decay is often occurring underneath the tooth, it is important we are able to recognize these subtle changes. The changes can occur on fresh tooth surface or around the margins of existing fillings.

MYTH- a black spot in a tooth is a hole- not necessarily- it might just be staining.

Probing the teeth

We may also gently probe any areas of your mouth that look suspicious with a bluntish probe looking for any stickiness or softness that may suggest caries in the tooth. This has been the normal way to check the fissues and patterns in your teeth for signs of decay for decades and has served me personally very well. In 99% of all the teeth that I found to be a little sticky, the decay had progressed into dentine. That said, care must be taken not to advance early problems by forcing the probe hard into the teeth and the saying ‘sharp eyes are better than sharp probes’ is a good thing for dentists to remember (from a great book called ‘The Fundementals of Operative Dentistry’) Check out ‘How does the dentist decide if I need a filling?’


This is a posh word for shining a light through your tooth in order to check for decay. The most common place it is used is on your front teeth which are thin enough for any caries in-between to be detected.

Taking X-rays- 

Your posterior teeth (back ones) however, are too thick for normal transillumination to be effective and require a much stronger light source which most dentists do not have. Because of this, we rely on X-rays to identify any caries that is going on in-between them, or around your existing fillings.

The best view for detecting general caries is the ‘reproducible bitewing radiograph’. Sometimes a peri-apical X-ray may be needed instead, or as well to check just how deep the decay has gone. Any changes around the root tip that may indicate a root canal treatment was needed.

We take X-rays if:

(i) We see anything that suggests decay; such as a grey edge of a tooth, or our probe gets stuck in between the teeth.

(ii) You report any symptoms of toothache or caries to try to identify the problem.

(iii) Where caries is obvious and we want to check how close it has progressed to the nerve- better we know up front the chance of a root canal treatment so we can inform you rather than stopping you half way through a procedure to say- “Surprise! If you want to keep your tooth you’ll need a root canal”.

(iv) It is time for screening X-rays (how often depends on your caries risk) where we often pick up ‘early’ and sometimes even’ late’ caries, if it hadn’t been giving you any symptoms.

For more details on when you need X-rays to check for decay, see screening xrays. The section on bitewing X-rays is also useful as this will show you how the dentist actually identifies decay and what it looks like.

What about New-Caries Diagnosing Technology?

The methods above will remain the classic ways for detecting dental caries for some years to come in your standard general practice. That said, we are seeing some exciting new technologies coming out, such as electronic caries detectors, ultrasonic devices and laser systems.

Whilst some are in general use in the US, they are not without their problems and remain quite technique-sensitive. If, before using these systems, any plaque (or even colour) remains in the tooth’s fissures, false (inaccurate) readings can occur. It is certainly a space to watch and I will keep you updated with developments.

What are the Signs or Symptoms that I have Dental Caries?

The dentist is responsible for diagnosing and treating dental caries, but they need to see you in order to be able to do this.

Here is a summary of the signs and symptoms you should be aware of which may suggest a dental visit is necessary:

  • Symptoms- (what you might experience)

(i) Sensitivity- to hot, to cold, to pressure or to sweet things

(ii) Toothache– A sharp or throbbing pain

(iii) Pain on biting or chewing

(iv) Symptoms of reversible and irreversible pulpitis

  • Signs- (what you may notice)

(i) A hole that you can see (dark spot) or that you can feel

(ii) Food always getting stuck between certain teeth

(iii) Floss that keeps tearing in a particular spot

(iv) A rough, chipped or fractured tooth.