2.1. Chapter objectives

This chapter describes

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the purpose of having case definitions for tuberculosis;

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the definition of a case of TB, as well as of suspected and confirmed cases;

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additional features of TB cases important for the treatment of individual patients, as well as for evaluating TB programmes and monitoring the epidemic.

The diagnosis of TB refers to the recognition by health workers (medical officer, nurse, paramedic or other) of an active case, i.e. a patient with current disease due to M. tuberculosis. The role of NTPs is different: they are responsible for ensuring that diagnosed cases are notified (1), meet the definition for case or definite case, and are treated appropriately, and that outcomes are evaluated.¹

All providers must report both new and retreatment TB cases and their treatment outcomes to local public health authorities, in conformance with applicable legal requirements (Standard 21 of the ISTC (5)). NTPs ensure that critical features of the TB case are recorded and reported so that treatment is appropriate and feedback is provided to the treating clinician (6). Analysis of these reports also helps the NTP manager to monitor trends and evaluate the effectiveness of TB activities at all levels.

2.2. Purposes of defining a TB case

Uniform criteria to define a TB case are needed for:

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proper patient registration and case notification;

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selecting appropriate standard treatment regimens (see Chapter 3);

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standardizing the process of data collection for TB control;

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evaluating the proportion of cases according to site, bacteriology and treatment history;

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cohort analysis of treatment outcomes;

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accurate monitoring of trends and evaluation of the effectiveness of TB programmes within and across districts, countries and global regions.

2.3. Case definitions

The TB case definitions below are based on the level of certainty of the diagnosis and on whether or not laboratory confirmation is available.

• Tuberculosis suspect. Any person who presents with symptoms or signs suggestive of TB. The most common symptom of pulmonary TB is a productive cough for more than 2 weeks,² which may be accompanied by other respiratory symptoms (shortness of breath, chest pains, haemoptysis) and/or constitutional symptoms (loss of appetite, weight loss, fever, night sweats, and fatigue).³
• Case of tuberculosis. A definite case of TB (defined below) or one in which a health worker (clinician or other medical practitioner) has diagnosed TB and has decided to treat the patient with a full course of TB treatment.
  Note. Any person given treatment for TB should be recorded as a case. Incomplete “trial” TB treatment should not be given as a method for diagnosis.
• Definite case of tuberculosis. A patient with Mycobacterium tuberculosis complex identified from a clinical specimen, either by culture or by a newer method such as molecular line probe assay. In countries that lack the laboratory capacity to routinely identify M. tuberculosis, a pulmonary case with one or more initial sputum smear examinations positive for acid-fast bacilli (AFB) is also considered to be a “definite” case, provided that there is a functional external quality assurance (EQA) system with blind rechecking.⁴

Cases of TB are also classified according to the:

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anatomical site of disease;

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bacteriological results (including drug resistance);

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history of previous treatment;

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HIV status of the patient.

Each of these key features of TB cases is discussed below.

2.4. Anatomical site of TB disease

In general, recommended treatment regimens are similar, irrespective of site (see section 8.2). Defining the site is important for recording and reporting purposes and to identify the more infectious patients – those with pulmonary involvement (who will be further subdivided by smear status – see section 2.5 below).

Pulmonary tuberculosis (PTB) refers to a case of TB (defined above) involving the lung parenchyma. Miliary tuberculosis is classified as pulmonary TB because there are lesions in the lungs. Tuberculous intrathoracic lymphadenopathy (mediastinal and/or hilar) or tuberculous pleural effusion, without radiographic abnormalities in the lungs, constitutes a case of extrapulmonary TB. A patient with both pulmonary and extrapulmonary TB should be classified as a case of pulmonary TB.

Extrapulmonary tuberculosis (EPTB) refers to a case of TB (defined above) involving organs other than the lungs, e.g. pleura, lymph nodes, abdomen, genitourinary tract, skin, joints and bones, meninges. Diagnosis should be based on at least one specimen with confirmed M. tuberculosis or histological or strong clinical evidence consistent with active EPTB, followed by a decision by a clinician to treat with a full course of tuberculosis chemotherapy. The case definition of an EPTB case with several sites affected depends on the site representing the most severe form of disease. Unless a case of EPTB is confirmed by culture as caused by M. tuberculosis, it cannot meet the “definite case” definition given in section 2.3 above.

2.5. Bacteriological results

Bacteriology refers to the smear status of pulmonary cases and the identification of M. tuberculosis for any case by culture or newer methods. Culture and drug susceptibility testing are discussed in section 3.8.1. For definitions of MDR-TB cases, see reference 7.

Standard 2 of the ISTC (5) states that all patients suspected of having pulmonary TB should submit at least two sputum specimens for microscopic examination in a quality-assured laboratory. When possible, at least one early-morning specimen should be obtained, as sputum collected at this time has the highest yield. ISTC Standard 4 states that all persons with chest radiographic findings suggestive of TB should submit sputum specimens for microbiological examination (5).

Smear-positive cases are the most infectious and most likely to transmit their disease in their surroundings; they are the focus for infection control measures (2) and contact investigations (3). Bacteriological monitoring of treatment progress is most feasible and practicable in these patients (see Chapter 4).

It is also important to identify smear-negative cases, especially in persons living with HIV for whom mortality is higher than in smear-positive pulmonary TB cases (4). For diagnostic algorithms for smear-negative persons living with HIV, see reference 4.

A case of pulmonary TB is considered to be smear-positive if one or more sputum smear specimens at the start of treatment are positive for AFB (provided that there is a functional EQA system with blind rechecking⁵).

The definition of a new sputum smear-positive pulmonary TB case is based on the presence of at least one acid fast bacillus (AFB+) in at least one sputum sample in countries with a well functioning EQA system. (See www.who.int/tb/dots/laboratory/policy/en/index1.html.)

Smear-negative PTB cases should either:

1. have sputum that is smear-negative but culture-positive for M. tuberculosis:
   • a case of pulmonary TB is considered to be smear-negative if at least two sputum specimens at the start of treatment are negative for AFB⁶ in countries with a functional EQA system, where the workload is very high and human resources are limited (see http:///www.who.int/tb/dots/laboratory/policy/en/index2.html);
   • in all settings with an HIV prevalence of >1% in pregnant women or ≥5% in TB patients, sputum culture for M. tuberculosis should be performed in patients who are sputum smear-negative to confirm the diagnosis of TB (4).
   OR
2. meet the following diagnostic criteria: (6–8)
   • decision by a clinician to treat with a full course of anti-TB therapy; and
   • radiographic abnormalities consistent with active pulmonary TB and
     either:

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     laboratory or strong clinical evidence of HIV infection

     or:

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     if HIV-negative (or unknown HIV status living in an area of low HIV prevalence), no improvement in response to a course of broad-spectrum antibiotics (excluding anti-TB drugs and fluoroquinolones and aminoglycosides).

Pulmonary TB cases without smear results are no longer classified as smear-negative (4); instead, they are recorded as “smear not done” on the TB register (6) and on the annual WHO survey of countries.

For patients suspected of having EPTB, specimens should be obtained from the suspected sites of involvement (Standard 3 of the ISTC (5)). Where available, culture and histopathological examination should also be carried out. Additionally, a chest X-ray and examination of sputum may be useful, especially in persons with HIV infection.

2.6. History of previous treatment: patient registration group

At the time of registration, each patient meeting the case definition is also classified according to whether or not he or she has previously received TB treatment and, if so, the outcome (if known). It is important to identify previously treated patients because they are at increased risk of drug resistance, including MDR-TB (see section 3.6). At the start of therapy, specimens should be obtained for culture and DST from all previously treated patients. Treatment depends on whether the patient has relapsed or is returning after default or after prior treatment has failed (see section 3.7). The distinctions between new and previously treated patients, and among the subgroups of previously treated patients, are also essential for monitoring the TB epidemic and programme performance.

New patients have never had treatment for TB, or have taken anti-TB drugs for less than 1 month. New patients may have positive or negative bacteriology and may have disease at any anatomical site.

Previously treated patients have received 1 month or more of anti-TB drugs in the past, may have positive or negative bacteriology and may have disease at any anatomical site. They are further classified by the outcome of their most recent course of treatment as shown in Table 2.1 below.

Table 2.1

REGISTRATION GROUP BY OUTCOME OF MOST RECENT TB TREATMENT.

Patients whose sputum is smear-positive at the end of (or returning from) a second or subsequent course of treatment are no longer defined as “chronic”. Instead, they should be classified by the outcome of their most recent retreatment course: relapsed, defaulted or failed.

2.7. HIV status

Determining and recording the patient's HIV status is critical for treatment decisions (see Chapters 3 and 5) as well as for monitoring trends and assessing programme performance. WHO's revised TB Treatment Card and TB Register include dates of HIV testing, starting co-trimoxazole, and starting ART. These important interventions are discussed more fully in Chapter 5.

References

1.

Engaging all health care providers in TB control: guidance on implementing public-private mix approaches. Geneva: World Health Organization; 2006. (WHO/HTM/TB/2006.360)

2.

WHO policy on TB infection control in health care facilities, congregate settings and households. Geneva: World Health Organization; 2009. (WHO/HTM/TB/2009.419) [PubMed: 24432438]

3.

Implementing the WHO Stop TB Strategy: a handbook for national tuberculosis control programmes. Geneva: World Health Organization; 2008. (WHO/HTM/TB/2008.40) [PubMed: 26269864]

4.

Improving the diagnosis and treatment of smear-negative pulmonary and extrapulmo-nary tuberculosis among adults and adolescents: recommendations for HIV-prevalent and resource-constrained settings. Geneva: World Health Organization; 2007. (WHO/HTM/TB/2007.379; WHO/HIV/2007.1)

5.

International Standards for Tuberculosis Care (ISTC). 2nd ed. The Hague: Tuberculosis Coalition for Technical Assistance; 2009.

6.

Revised TB recording and reporting forms and registers – version 2006. Geneva: World Health Organization; 2006. (WHO/HTM/TB/2006.373; available at: www​.who.int/tb/dots/r_and_r_forms​/en/index.html)

7.

Guidelines for the programmatic management of drug-resistant tuberculosis: emergency update 2008. Geneva: World Health Organization; 2008. (WHO/HTM/TB/2008.402)

8.

Global tuberculosis control 2009: epidemiology, strategy, financing. WHO report 2009. Geneva: World Health Organization; 2009. (WHO/HTM/TB/2009.411)

Footnotes

1

NTP programmes also facilitate the detection of cases via sputum screening of suspects with cough attending health facilities (2), as well as screening of contacts (3) and screening of persons living with HIV/AIDS (4).

2

Standard 1 of the International Standards for TB Care (5) states that all persons with otherwise unexplained productive cough lasting 2–3 weeks or more should be evaluated for TB.

3

The definition of a “TB suspect” depends on other local factors, including the patient's age and HIV status, HIV prevalence in the population, TB prevalence in the population, etc.

4

See: www​.who.int/tb/dots/laboratory​/policy/en/index1.html.

5

In countries without functional EQA, the definition from the third edition of these guidelines applies: a smear-positive pulmonary TB case was defined as one with:

1. two or more initial sputum smear examinations positive for AFB, or
2. one sputum smear examination positive for AFB plus radiographic abnormalities consistent with active PTB as determined by a clinician, or
3. one sputum smear positive for AFB plus sputum culture-positive for M. tuberculosis.

6

And no specimen is smear-positive.