| Introduction |
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The district-level TB surveillance objective is to follow trends in pulmonary TB cases and indicators of complete treatment (which will render the patient non-infectious). |
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There are two main forms of the disease tuberculosis, namely pulmonary TB and extra pulmonary TB. |
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The pulmonary form of the disease, which spreads predominantly by droplet infection through coughing or sneezing, tends to be the infectious form. It is easily transmitted from person-to-person. |
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Extra-pulmonary TB that affects all other human organs apart from the lung tissue is rarely infectious. |
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From a public health perspective pulmonary TB is the primary focus for interventions. Among the pulmonary TB form, there are those with high bacillary load which are easily detected by light microscopic examination of sputum specimens (known as smear-positive PTB), and those with relatively low bacillary load which are not so easily detected by light microscopy (known as smear-negative PTB). Smear-positive pulmonary TB is more infectious than smear-negative pulmonary TB. |
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On average, a single smear-positive pulmonary TB case leads to 10-15 new infections in a year and 5-7 new clinically apparent pulmonary TB cases within a year. |
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HIV will cause the number of TB cases in a district to increase, even at relatively low levels of HIV infection in the district. In fact, the epidemiology of TB cases in a district may be a good indicator of the level of HIV, the geographic spread, and whether the HIV epidemic is concentrated in high risk populations or has spread widely in the general population.
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| Laboratory analysis |
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Diagnosis of TB is based on bacteriological examination of sputum specimens for Acid Fast Bacilli (AFB) stained by the Ziehl-Neelsen method, using a light microscope. Three sputum specimens from a TB suspect, collected over two consecutive days must be examined. |
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By definition, a new smear-positive pulmonary TB case is diagnosed when one of the following two scenarios emerge: 1) two positive smear results (out of the 3), 2) one positive smear result supported by suggestive radiological evidence, and a decision by a Medical Officer to treat with a complete course of anti-TB chemotherapy. The positive predictive value of a diagnosis of new smear-positive pulmonary TB using the two criteria above is approximately 80% for patients with highly infectious pulmonary TB. The sensitivity and specificity values for sputum smear microscopy are generally high (>70%).
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| Analysis of time, place, and person. |
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New Case Notification Classifications. |
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Approximately 65% of all pulmonary cases registered in a quarter should be smear-positive. |
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If the percentage of smear-positive cases is significantly lower, the quality of diagnosis of pulmonary TB may be poor. |
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There should be approximately a 1 to 1 relationship between the number of new smear-positive cases and the number of new smear-negative cases and extra-pulmonary cases combined. Should this ratio be grossly distorted, then the quality of diagnosis may again be suspect. For example, extra-pulmonary or smear-negative pulmonary TB may be over-diagnosed. |
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If there is an increasing number of extra-pulmonary TB cases (in the absence of increasing over-all TB or smear-positive pulmonary cases), one should also consider an increase in HIV since HIV causes slightly more extra-pulmonary cases than pulmonary cases. With HIV both extra-pulmonary and pulmonary will increase, but extra-pulmonary may increase slightly faster. |
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If the ratio of smear-negative cases plus extra-pulmonary cases to smear-positive cases is too high, one should examine the quality of laboratory diagnosis. |
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A decreasing trend of smear-positive PTB cases could indicate less reliance on smear microscopy for diagnosis leading to over-diagnosis of smear-negative and extra-pulmonary TB. For instance, there could be an over-diagnosis of smear-negative cases by the overuse of radiological examination by a clinician who is not familiar with the recommendations of the national TB programme guidelines. |
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If there is an increasing number of smear-negative pulmonary cases (in the absence of increasing over-all TB or smear-positive pulmonary cases), the district team should consider: a problem with lab--falsenegative smears, and over-use or over-reading of x-rays or other methods of diagnosing pulmonary TB. |
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Category of Retreatment cases |
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Under a well-performing program, the proportion of registered TB cases that result in relapse, failures, or retreatment after interruption should remain stable or decline with time. |
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An increasing proportion of registered TB cases that result in retreatment usually indicates a decline in the performance of the TB program. An increasing proportion of registered TB cases that result in relapse can indicate increasing drug resistance. |
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Age and gender of new smear-positive pulmonary cases. |
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In the African Region, most cases occur in the 15-49 age group. Under the age of 15 years, the prevalence of disease is the same in both males and females. However, after 15 years, more males are affected than females. Thus, if the sex ratio before 15 years in a district is high in favor of males it may indicate poor accessibility of TB services for female children. |
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Where TB control is effective, the age distribution of TB cases shifts to older age groups. |
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Changes in age distribution of TB cases (increases in 15-24 year old females) may reflect the epidemiology of HIV in the district. |
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Cohort Analysis of Treatment Results |
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In calculating the cohort analysis of treatment results, the denominator is all registered patients. The percentage of registered patients that are evaluated and the percentage of all registered patients that are smear negative at the end of treatment (cured) are important indicators that the district should monitor. |
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The percentage of registered patients that are cured should increase to 85%. It may be difficult to reach 85% "cured" in districts with high rates of HIV since many patients in each treatment cohort will die. |
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An increasing proportion of patients registered but not evaluated, with failure or who interrupted treatment, is a warning sign indicating potential program problems. |
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Case fatality ratio |
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In the absence of HIV infection, the case fatality ratio is approximately 0.15, but this is higher (0.20-0.30) in the presence of HIV infection. If case fatality increases, one should consider increasing HIV infection, increasing TB drug resistance, or poor program performance as the cause.
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| Public health action and targets. |
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For control of TB in the African Region, WHO recommends the DOTS strategy (Directly Observed Therapy, Short-course). This is a cost-effective strategy based on early case detection primarily through microscopic diagnosis of sputum specimens and early treatment using a standardized combination of antimicrobials administered over a relatively short period (e.g. six months) under direct observation by a trained treatment supporter. When correctly applied, this ensures cure of infectious cases (and other cases) and thereby reduces the transmission of the disease. |
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For individual patients, DOTS treatment results in a cure rate of >90% when correctly given. |
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DOTS can result in a 10% decline in smear-positive PTB cases per annum in a district in the absence of HIV infection. |
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The first priority of every TB program must therefore be to direct resources towards identifying the sick infectious cases (smear-positive pulmonary TB cases) so they can be cured. Correct implementation of DOTS in a district should result in cure rates of 80-85%. |
