The art and science of diagnosing apical periodontitis in root-filled teeth are hampered by several difficulties. Misdiagnosis can therefore not be completely avoided. A deliberate strategy may be a mean to steer away from unwanted mistakes or to guide to acceptable and calculated mistakes. If a clinician, in a given situation, wants to minimize the number of false-positive diagnoses, i.e., wants to minimize the number of healthy wrongly diagnosed as being ill, the requirements for a positive diagnosis should be kept strict, and a positive diagnosis should be reported only when absolutely certain. Adopting such a strategy, the patient or the tooth “benefits of the doubt.” If, on the other hand, the clinician wants to minimize the number of false-negative diagnoses, i.e., wants to minimize the number of sick registered as healthy, the requirements for a positive diagnosis should be more including, and a positive diagnosis should be reported whenever there is a suspicion of disease. Such a strategy in contrast to above could be characterized as “trap rather than free.” A reduction of false-positives diagnoses always brings an increase in the number of false-negative diagnoses and vice versa. Selection of strategy thus always has a desired and an undesired effect [33, 51].

It is likely that a more general use of CBCT would improve the diagnostic accuracy of apical periodontitis in root-filled teeth. In particular, the proportion of false-negative diagnoses could be reduced. But it cannot be ruled out that the risk of an increased amount of false-positive diagnoses could also be the result, in particular in the absence of symptoms and when then the preoperative appearance of the lesion is unknown. Also, because the dynamics of the healing process over time after root filling is undetermined, unrestrainedly use of CBCT would bring about risk of substantial overdiagnosis and as logical result also overtreatment (for further reading, see Chap. 6 “Decision Making”). As with any ionizing radiation exposure to patients, the potential benefits should outweigh the potential risks.

The high frequency of root-filled teeth with periapical bone destructions seems to persist despite the technical quality of root fillings has improved over time [52, 53].

Millions of teeth saved to survival and asymptomatic function are present in many countries all over the world. The risks of a systemic adverse effect on the health of untreated apical periodontitis may, on the basis of the evidence currently available, considered small for healthy patients (see Chap. 4 “Consequences”). Severe acute infectious condition resulting from apical periodontitis in root-filled teeth is also unusual and has been estimated to less than 5% over a period of 25 years [54]. Unfortunately, there is no scientifically established method to distinguish between different severities of periapical disease. There is some evidence that there is a connection between the periapical size and the amount of involved microorganisms [21]. Many different microorganisms, in most cases the bacteria but sometime fungi, have been found in the biofilms that persist in filled root canals. There are no accepted scientifically founded clinical methods to distinguish particularly “dangerous” or “harmless” biofilms from this perspective.

In many cases the root filling quality is poor, and an apical lesion is apparent to anyone who is observing the radiograph despite the fact the patient is free from symptoms. In such cases there is little or no doubt about the diagnosis; the tooth should be diagnosed with persistent apical periodontitis. In other situations, the periapical radiolucency is big and shows no signs of reduction in size despite adequate root canal treatment. It may be suspected that the periapical lesion represents a periradicular cyst without any further healing potential without further treatment [55, 56]. The next steps are to inform the patient and have a dialogue about how the situation should be managed. This process is reviewed in Chap. 6.

However, if the root filling quality is reasonable within acceptable standards, the clinician may remain ambiguous about the periapical diagnosis.

When uncertain about the diagnosis of apical periodontitis, it seems likely from the bulk of available information that most patients will “benefit from the doubt” when apical periodontitis in an asymptomatic and properly root-filled tooth is considered. In other words, false-positive diagnoses should be avoided. This means that the clinician deliberately should choose to refrain from diagnosing apical periodontitis in root-filled teeth when in doubt, rather than taking risk of diagnosing and hence treating teeth with healed or healing apical periodontitis.

A careful medical history is important for all patients under dental care. There are several medical conditions and medications that cause a deterioration of the immune system, by the lack of white blood cells or the inability of a patient to produce antibodies.

In such situations, maybe that “trap rather than free” attitude by the clinician may be the best diagnostic strategy. However, it is unclear to what extent medically comprised patients benefit from diagnoses and in particular treatment of asymptomatic apical periodontitis in a situation of ongoing medications or disease that impede normal function of the immune defense system.

In the following we will give some typical examples where there may be an uncertainty regarding potential residual apical periodontitis in a root-filled tooth. We are also suggesting a concrete policy reaching a diagnosis in each one of the situations.

When the diagnosis of apical periodontitis is made, it is the dentist’s statutory obligation to inform about the diagnosis and demonstrate the potential of therapy. The dentist should also give suggestions on the treatment that he/she deems most appropriate. In addition, the dentist must inform if the treatment is requiring a referral to a specialist or not. It is the patient who must take the final decision to treat or not. This process will be further covered in Chap. 6.