|Tuberculosis: Review of PPD Testing and Prophylaxis|
Tuberculosis: Review of PPD Testing and Prophylaxis
Baldeep Singh, M.D.
After a concerning increase in the number of tuberculosis (TB) cases early in the 90\'s, the trends in TB in the United States have been slowly improving. In 1998, 18,361 cases of TB were reported in the United States (6.8 cases per 100,000 population) - an 8% decline in cases from 1997 and a 31% decline in cases since 1992.1 In California specifically, there were 3,852 cases in 1998, a 28% decline from 1992.
The declining national and state-wide statistics are directly related to the apparent strengthening of TB control programs since 1992. Much of the decline can also be attributed to the change in recommendations in the arena of prophylactic therapy for TB. However, the Center for Disease Control goal of decreasing TB\'s prevalence to 1 case in 100,000 by the year 2010 will probably not be achieved given the current rate of decrease. Therefore, the medical community must continue aggressively screening and treating patients for TB.
The following article is a summary of the most recent recommendations regarding TB prophylaxis. This is not a guideline for the treatment of active TB.
The tuberculin skin test is the only available method for establishing the diagnosis of M. tuberculosis infection in the absence of clinical symptoms. The 5 tuberculin units (TU), or intermediate strength tuberculin, applied by the Mantoux (intradermal) technique should always be used.
The definition of a positive test depends on one\'s risk factors. One must measure the amount of induration, not erythema, on the transverse axis of the arm and document it as one measurement in millimeters.
The American Thoracic Society and Centers for Disease Control (ATS/CDC) recommends the following interpretation of the tuberculin test with three defined cutoff values for significant reactions (or positive tests):2
The PPD is read 48 to 72 hours after placement of the test. Reading should be based on palpating the skin for induration borders. The reading should not be based on the extent of erythema on the arm. Many providers use a pen to facilitate measurement of induration. A single measurement of the induration diameter transverse to the long axis of the forearm should be recorded (eg. not a reading of 12 by 8 mm). A positive test is based on the risk factor recommendations above.
The Booster Phenomenon
Although boosting occurs in all age groups, it increases substantially with age and is most frequently encountered among persons older than 55 years of age. The difficulty of defining criteria for boosted reactions and new converters reflects the imperfections of the tuberculin test.
A two-step tuberculin skin testing protocol is recommended for all persons who will undergo serial tuberculin skin tests. When tuberculin skin testing of adults is to be repeated periodically, the use of two-step testing initially can reduce the likelihood that a boosted reaction will be interpreted as representing recent infection. It is important to do this with the initial PPD placement so that subsequent testing is not misinterpreted as a recent conversion.
If the reaction to the first test is negative, a second test should be given a week later. If the second test result remains below the cut point for a positive test, the reaction is considered to be negative. If the reaction to the second test is positive, it probably represents a boosted reaction and not a new infection. If this important step is not done, and the patient is tested the following year or two, the result may mistakenly be considered a new conversion, and thus treated inappropriately.
Past BCG Innoculation
A prior history of vaccination with the Bacille Calmette-Gu?rian (BCG) is frequently cited as a reason not to perform a tuberculin skin test, because of problems interpreting the result. However, studies estimate that this tuberculin sensitivity wanes at a rate of 10% per year unless repeated doses of BCG (or tuberculin) are administered annually.
After 10 years, the rate of false positives from BCG is quite low. Therefore, the CDC recommends ignoring a history of BCG vaccination in deciding whether to apply the tuberculin test or how to interpret it.2
False negative tuberculin tests occur because of acute infections, live viral vaccines, poor nutritional status, metabolic derangements, immunosuppressive therapy, and diseases affecting the lymphoid tissue. Factors related to tuberculin used (storage, dilution, contamination, adsorption) and to the method of administration may also explain false negative tuberculin tests.
Application of control skin antigens to determine anergy in ambulatory patients tested with PPD is not routinely recommended, with the exception of patients with HIV.
The rate of false negative tuberculin tests among persons infected with HIV depends on the severity of immunosuppression. In patients presenting with AIDS and TB at the same time, approximately 33%-50% will have positive tuberculin tests at the time of diagnosis. However, most persons with asymptomatic HIV infection and TB will have a positive tuberculin test.
The CDC recommends that all HIV infected persons be evaluated for delayed type hypersensitivity anergy at the time of tuberculin testing. Application of at least two control antigens (e.g. candida, mumps or tetanus toxoid) by the Mantoux method is recommended. Any induration measured at 48-72 hours is considered evidence of a positive response.
Positive PPD Conversion to Active TB
Overall, between 5% and 15% of all tuberculin reactors will develop active TB during their lifetime. Thus, the decision to treat a patient is based on their cumulative risk of developing active TB versus their risk of developing an adverse reaction to isoniazid.
The rates of conversion to active to TB are listed in Table 1.
These data are the basis for the recommendation regarding treatment listed below.
Preventive Therapy Efficacy
There is strong evidence to support the effectiveness of isoniazid therapy. In patients with a positive PPD and a negative chest X-ray, a consistent reduction in cases of TB was demonstrated by using isoniazid for six months. Six months of therapy was chosen after data revealed that this course conferred a 65% reduction in cases reported compared with 70% reduction at
one year of therapy2. Given the clear improvement in patient compliance, the CDC recommendation was changed in 1993 to six months from one year.
Risks of Isoniazid
Elevations of serum hepatic enzymes (AST or ALT) occur in 10% to 20% of persons on isoniazid. Clinical hepatitis is infrequent, and is directly related to increasing age. The age cut off of 35 was chosen based on the risk of isoniazid hepatitis in the low risk groups.
The risk of hepatitis is highly correlated with alcohol intake. However, persons who abuse alcohol are also at increased risk of TB, and this should not be used as a reason to not recommend isoniazid therapy. Asymptomatic chronic carriers of HBsAg do not have an increased risk of isoniazid hepatitis, and so this is not a contraindication to prophylactic therapy.
Routinely following liver function tests is not recommended in low risk groups. However, serial hepatic enzyme testing is recommended in patients over age 35 and in high risk groups including alcohol users, IV drug users, and those with chronic liver disease. Additionally, there is evidence that minority women who are post-puberty are also at higher risk and should receive closer monitoring. isoniazid should NOT be used until at least six months post-partum if a pregnant woman is found to be PPD positive.
Side effects from isoniazid include: somnolence, difficulties with concentration, dizziness and gastrointestinal upset. Allergic rashes occur infrequently.This may be improved by taking the medication before bed and/or with food. Isoniazid interferes with the metabolism of phenytoin, leading to an increase in the serum levels.
Use of Vitamin B6
Peripheral neuropathy occurs in <1% of persons on isoniazid, and is preventable with pyridoxine (vitamin B6) supplements of 10 to 25 mg/day. Routine pyridoxine is not recommended. The following risk groups are recommend for adjunctive pyridoxine therapy: Persons older than 65 years of age, pregnant women, diabetics, daily alcohol users, patients with chronic renal failure, poorly nourished persons and anyone with any other predisposition to peripheral neuropathy.
Initiating and Monitoring Therapy
Isoniazid is administered daily in a dose of 300 mg/day. A chest X-ray showing no abnormalities consistent with active TB is mandatory prior to initiating treatment. If radiographic abnormalities are present, active disease should be ruled out with sputum cultures. One or two calcified granulomas do not usually qualify as a significant TB abnormality.
Under Age 35
Anyone under the age of 35 who meets criteria should be given isoniazid therapy unless contraindicated. Baseline liver function tests are not recommended for individuals younger than 35 years of age unless there is a history of alcohol abuse or liver disease. However, the CDC does recommend monthly evaluations of patients to monitor for adverse side effects. If the patient develops symptoms compatible with hepatitis, the isoniazid should be stopped until liver function tests can be obtained. Asymptomatic persons with AST greater than 3 - 5 times the upper limits of normal should also discontinue isoniazid. The reported deaths from isoniazid hepatitis have occurred when the patients were continued on the drug through jaundice.
Women, especially during and following pregnancy, may be at increased risk of isoniazid -associated hepatitis. It is, therefore, not recommended to give isoniazid during pregnancy or up to six months postpartum to avoid this potential risk.
Over Age 35
If the patient is over age 35, the guidelines to give preventive therapy are much stricter. The criteria are as follows:
The CDC does recommend monthly liver function tests in patients over 35 since the incidence of isoniazid induced hepatitis is higher.
The CDC makes different recommendations in two specific categories:
Preventive Therapy Regimens for Adults with HIV Infection
The CDC recently revised the guideline for TB prophylaxis in patient with HIV.4 The major change was to reduce the recommendation for isoniazid therapy from 12 months to nine months. Two other recommended regimens are listed on the following page. Patients must carefully adhere to the strict guidelines, paying special attention to their antiviral and antibacterial prophylactic regimen.
In conclusion, TB prophylaxis is an effective and underused therapy. Adherence to the CDC consensus guidelines would increase the number of patients treated. Los Angeles is now considered a high risk environment, and thus stricter adherence to treatment guidelines should be taken. If we are to continue the encouraging trend towards TB eradication in the United States, an aggressive approach to the TB prophylaxis is warranted.