Diagnosis
Tuberculin Skin Test (TST) and Interferon-Gamma Release Assays (IGRAs)
- The Tuberculin Skin Test (Mantoux test) involves intradermal injection of purified protein derivative (PPD) with measurement of induration (not erythema) at 48-72 hours. Interpretation depends on the patient's risk factors, with cutoffs at 5mm, 10mm, or 15mm.
- IGRAs (QuantiFERON-TB Gold, T-SPOT.TB) measure T-cell release of interferon-gamma in response to TB-specific antigens. These tests are not affected by prior BCG vaccination and require only one patient visit.
| TST Induration Interpretation |
Patient Category |
| ≥5mm is positive |
HIV-positive, recent TB contacts, organ transplants, immunosuppressed patients, patients with fibrotic changes on chest X-ray |
| ≥10mm is positive |
Recent immigrants from high-prevalence countries, injection drug users, residents/employees of high-risk settings, children <4 years, persons with clinical conditions increasing TB risk |
| ≥15mm is positive |
Persons with no known risk factors for TB |
Key Points
- Both TST and IGRAs indicate infection but cannot distinguish between latent and active TB.
- IGRAs are preferred for BCG-vaccinated individuals to avoid false positives.
Radiographic and Laboratory Diagnosis
- Chest X-ray findings in TB include upper lobe infiltrates, cavitation, nodular opacities, and hilar or paratracheal lymphadenopathy. In primary TB, lower or middle lobe infiltrates with hilar lymphadenopathy are common, while reactivation TB typically shows upper lobe involvement with cavitation.
- Definitive diagnosis requires identification of M. tuberculosis in clinical specimens, typically through acid-fast bacilli (AFB) smear, culture, or nucleic acid amplification tests (NAATs). Cultures remain the gold standard but take 2-6 weeks, while NAATs provide results within hours.
Negative AFB smears do not rule out TB. Up to 50% of culture-confirmed TB cases have negative smears.
Key Points
- Three sputum specimens collected on separate days improve diagnostic yield.
- Drug susceptibility testing is essential for all positive cultures to guide therapy.
Treatment
Medication Regimens
- The standard treatment for drug-susceptible TB consists of a four-drug regimen during the intensive phase: isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB) for 2 months, followed by INH and RIF for 4 months in the continuation phase.
- Treatment for multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) requires longer durations (18-24 months) and includes second-line drugs such as fluoroquinolones, injectable agents, and newer medications like bedaquiline.
Memory Aid for First-Line TB Drugs: "RIPE"
Rifampin - Orange body fluids, P450 inducer
Isoniazid - Peripheral neuropathy (give with B6)
Pyrazinamide - Hyperuricemia, hepatotoxicity
Ethambutol - Optic neuritis (red-green color discrimination)
Key Points
- Directly Observed Therapy (DOT) is recommended to ensure adherence and prevent drug resistance.
- Baseline and periodic monitoring of liver function, vision, and kidney function is essential during treatment.
Medication Side Effects and Monitoring
- Isoniazid can cause peripheral neuropathy (prevented by pyridoxine/vitamin B6 supplementation), hepatotoxicity, and drug-induced lupus. Rifampin causes orange discoloration of body fluids, hepatotoxicity, and is a potent inducer of cytochrome P450, leading to numerous drug interactions.
- Ethambutol may cause optic neuritis with decreased visual acuity and red-green color discrimination, while pyrazinamide can cause hepatotoxicity, hyperuricemia, and arthralgias. Baseline and monthly monitoring varies by medication but includes liver function tests, visual acuity, and symptom assessment.
Instruct patients to immediately report symptoms of hepatitis (nausea, vomiting, abdominal pain, jaundice, dark urine) and to stop medications until evaluated.
Monitoring Protocol for TB Treatment
- Obtain baseline liver function tests, complete blood count, creatinine, and uric acid for all patients.
- Test visual acuity and color discrimination at baseline and monthly for patients on ethambutol.
- Monitor liver function tests monthly for patients with baseline abnormalities, HIV, pregnancy, or alcohol use.
- Assess adherence, side effects, and symptom improvement at each visit.
- Collect sputum for AFB smear and culture at least monthly until cultures convert to negative.
Key Points
- Patients should take pyridoxine (vitamin B6) with isoniazid to prevent peripheral neuropathy.
- Rifampin reduces the effectiveness of hormonal contraceptives; recommend alternative birth control methods.
Special Populations
TB and HIV Co-infection
- HIV infection is the strongest risk factor for progression from latent to active TB. TB-HIV co-infection presents diagnostic challenges as patients may have atypical presentations, including normal chest X-rays, negative skin tests, and extrapulmonary disease.
- Treatment principles include early initiation of antiretroviral therapy (ART), typically within 2-8 weeks of starting TB treatment, while monitoring for immune reconstitution inflammatory syndrome (IRIS). Drug interactions between rifampin and certain antiretrovirals require dose adjustments or alternative regimens.
TB-HIV co-infected patients have higher mortality rates and require close monitoring for drug interactions, overlapping toxicities, and IRIS.
Key Points
- All TB patients should be tested for HIV, and all HIV patients should be screened for TB.
- Rifabutin may be substituted for rifampin to minimize drug interactions with antiretrovirals.
Pediatric and Pregnancy Considerations
- Children with TB often present with non-specific symptoms and are more likely to develop disseminated disease, including TB meningitis and miliary TB. Diagnosis is challenging due to difficulty obtaining sputum samples and paucibacillary disease (fewer bacteria).
- TB during pregnancy increases risks of maternal mortality, preterm birth, low birth weight, and perinatal mortality. The standard four-drug regimen is considered safe during pregnancy, with the exception of streptomycin. Pyridoxine supplementation is particularly important for pregnant women on isoniazid.
Key Points
- Gastric aspirates, induced sputum, or bronchoalveolar lavage may be needed to diagnose TB in young children.
- Pregnant women with TB should receive treatment without delay, as the benefits outweigh the risks.
Study Tips
Memory Aids
TB Risk Factors: "HIT TB"
HIV/immunosuppression
Immigration from endemic areas
Time spent in high-risk settings (prisons, homeless shelters)
TB exposure history
Biological factors (diabetes, malnutrition, silicosis, end-stage renal disease)
Classic TB Symptoms: "WEIGHT"
Weight loss
Evening fevers
Incessant cough
General fatigue
Hemoptysis
Thoracic pain
Key Points
- Focus on the distinguishing features of TB from other respiratory conditions, particularly the gradual onset and chronic nature of symptoms.
- Understand the significance of risk factors in determining the pre-test probability of TB infection.
Common Pitfalls
- Misinterpreting TST results: Remember that erythema is not measured, only induration, and interpretation depends on risk factors.
- Confusing isolation requirements: TB requires airborne precautions (negative pressure room, N95 respirator), not droplet or contact precautions.
- Overlooking extrapulmonary TB: TB can affect any organ system; maintain high suspicion in patients with risk factors and unexplained symptoms.
- Medication interactions: Rifampin has numerous drug interactions due to cytochrome P450 induction, including reducing the effectiveness of oral contraceptives, warfarin, and many antiretrovirals.
Never add a single drug to a failing TB regimen, as this promotes resistance. Always add at least two new drugs to which the organism is likely to be susceptible.
Key Points
- Prioritize questions about high-risk populations, infection control, and medication side effects on the NCLEX.
- Practice applying nursing process to TB scenarios, particularly assessment findings and appropriate interventions.
Self-Assessment
Quick Check: TB Knowledge
1. What is the most common route of TB transmission?
2. Name the four first-line drugs for TB treatment.
3. What type of isolation is required for infectious TB patients?
4. What is the minimum duration of treatment for drug-susceptible pulmonary TB?
5. What vitamin should be given with isoniazid and why?
Self-Assessment Checklist