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A caries assessment device should fulfil some prerequisites before it can be applied clinically. These include: manageability (be cheap, fast, acceptable, and easy to learn), reproducibility (ability to show the same results when a sample is measured twice or more by the same observer), and accuracy (to be able to detect and determine whether a disease is truly present or not, and whether the codes and descriptions are unambiguously presented) [4]. To what extent commonly applied visual/tactile devices have been tested for the presence of these prerequisites is worth investigating. Validity testing is particularly important and not always performed correctly [5, 6]. The following paragraphs introduce some visual/tactile devices currently in use.

While it is accepted that the explorer should not be used to detect carious lesions, as damage can occur to enamel when pushing the explorer into pits and fissures and in brittle enamel carious lesions, the explorer is part of the assessment of the level of “activity” of the lesion. Activity is assessed by removing biofilm from stagnation areas with the explorer and to “sense” the roughness and hardness of the lesion as the tip of the instrument is drawn gently across the lesion. Nyvad et al. [7] described activity assessment and developed a carious lesion activity assessment system. In addition to assessing various stages of carious lesions in enamel and dentine, this system also includes an evaluation of restorations but not for teeth missing due to dental caries. The Nyvad system appears to have predictive validity on the basis of a single study. However, whether a carious lesion was active or inactive at the start of the intervention that tested the predictive value had no influence on the outcome after 3 years [8].

The International Caries Detection and Assessment System (ICDAS) Collaboration Group developed and promoted the ICDAS 2-digit index as a new classification system in caries epidemiology by emphasising the importance of assessing various stages of enamel carious lesions in reaction to the reduction of dentine carious lesions in a number of western countries [9]. However, many visual/tactile indices that contain enamel carious lesions codes have been introduced since the mid-1950s [10] and provide useful information with respect to the suitability of more than 1 code for the reliable assessment of enamel carious lesions [11]. ICDAS was upgraded to ICDAS II, followed a couple of years later by ICDAS II-PUFA, and most recently by the International Caries Classification and Management System (ICCMS) [12]. The presence of face, content, and construct validity of ICDAS II has been questioned [13]. Researchers who used ICDAS (II) have altered the system over the years because of difficulties encountered when using it in epidemiological surveys in the field [14–16]. Another limitation in using ICDAS relates to reporting results. Initially, the DMFT/S unit was used to report results, followed later by the DMFT/S+PUFA modification, and later again by the DMFT/S_ICDAS/LA where “LA” stands for “lesion activity.” To be trained and calibrated in 3 differentiating codes for carious lesions in enamel, and then to group these codes together with code 0 to produce a dmf/DMF score seems like a great deal of additional work to end up with a simplified coding system that could have been used all along. After testing 2 codes for recording stages in a carious lesion in enamel, Marthaler [11] concluded that 1 code is sufficient. With the introduction of the ICCMS in 2013 [12], the ICDAS II index became split into a care index (former 1st digit) and a carious lesion index (former 2nd digit), with various options to merge the 7 caries-related codes for reporting results. With many changes in ICDAS over a relatively short period of time, it is difficult to see whether each of these steps, and upgraded steps, have been validated, especially in an epidemiological setting. Therefore, one should think twice before using ICDAS (II) or ICCMS in epidemiological surveys.

Around 2010, the pulp/ulceration/fistulae/abscess (pufa/PUFA) index was introduced [17]. The index assesses the pathological consequences of the caries process only and appears to be a valuable addition to recording caries-related conditions in epidemiology. It does not assess enamel or dentine carious lesions that can be restored, neither does it determine the presence of restorations or teeth lost due to dental caries. A few studies have been published and none have reported the validity of the index.

The index used most frequently in caries epidemiological surveys worldwide is that developed by the World Health Organisation (WHO) [18]. The index basically assesses whether or not a carious lesion is cavitated into dentine. Teeth missing due to dental caries and tooth restorations are also recorded.

The visual/tactile indices described above each have their limitations for use in caries epidemiological surveys. For example, the pufa/PUFA index needs to be complemented by an index that assesses enamel and dentine carious lesions without pulpal involvement; the Nyvad criteria needs the addition of a “missing teeth due to caries” category, the ICDAS II (double digits) needs the pufa/PUFA index, while the WHO index needs to be supplemented with an index that includes enamel carious lesions and the pufa/PUFA index.

The disadvantages of the visual/tactile indices described above are overcome by a newly developed assessment instrument termed the “Caries Assessment Spectrum and Treatment” (CAST) instrument [19]. It was introduced for the assessment of dental caries-related conditions and treatment in epidemiological surveys and designed to overcome the shortcomings of the indices/systems described above. It permits the registration of sound teeth, sealants, restorations, enamel, and dentine carious lesions, advanced stages of carious lesions into the pulp and tooth-surrounding tissues, and teeth lost from dental caries (Table 1). The assessment is performed visually, with the naked eye, and does not use compressed air for drying tooth surfaces. CAST consists of 10 codes that are ordered hierarchically. This implies that a sealant (code 1) is less severe than an enamel carious lesion (code 3), and that a dentine carious lesion that can be restored (code 5) is less severe than a tooth with a carious lesion with pulpal involvement (code 6).

Research showed that the CAST instrument has face, content, construct, and external validity for use in children and adults [20, 21] and has a high level of reproducibility [22]. The CAST codes can be converted to dmf/DMF counts so that dmf/DMF scores can be compared with those obtained from using the WHO index [23]. A restoration (code 2) and a tooth lost due to dental caries (code 8) is considered not diseased. With CAST, a caries-diseased tooth is one that has a dentine carious lesion (codes 4, 5) or has pathology (code 6, 7). This affects the determination of the prevalence of dental caries in a population, as discussed in the next section.

The CAST instrument needs to be tested in populations of different ages and backgrounds than those studied so far. CAST has been used or is in use in epidemiological surveys in Brazil [23], India [24], Pakistan [25], Poland [26], Mozambique, Peru, Russia, Surinam, and Turkey.