🫁 TB Directly Observed Therapy Adherence Simulator

Diagnosis & Treatment Initiation — Starting the RIPE Intensive Phase

Tuberculosis remains one of the world's leading infectious killers. Once active pulmonary TB is confirmed — by sputum smear microscopy, Xpert MTB/RIF molecular testing, or culture — treatment begins the same day with a standardized four-drug regimen designed to rapidly kill the large, fast-dividing bacterial population and prevent the emergence of resistant mutants from the outset.

• 10.8 M: Global TB cases (2023) (WHO Global TB Report)
• 4: Intensive-phase drugs (Rifampin · Isoniazid · Pyrazinamide · Ethambutol)
• 6 months: Standard course length (26 weeks, drug-susceptible TB)
• ~95–98%: Cure rate with full adherence (when doses are fully observed)

Why four drugs, and why immediately

A single actively replicating TB lesion can contain 10⁷–10⁹ Mycobacterium tuberculosis bacilli. Spontaneous resistance mutations to any one drug arise at a rate of roughly 1 in 10⁶–10⁸ divisions. With a population that large, resistant mutants to a single drug are essentially guaranteed to already exist before treatment starts.

Using four drugs simultaneously — each targeting the organism through a different mechanism — makes it statistically near-impossible for a bacillus to be simultaneously resistant to all four. Rifampin and isoniazid are the most potent sterilizing and bactericidal agents; pyrazinamide uniquely kills bacilli in the acidic, semi-dormant intracellular environment; ethambutol suppresses the emergence of resistance while the diagnosis (and any pre-existing resistance) is still being confirmed by culture.

Starting the correct four-drug combination on day one — before drug-susceptibility results return — is what keeps a curable infection from drifting toward multidrug-resistant TB (MDR-TB).

From symptoms to confirmed diagnosis

Typical presentation includes cough lasting more than two weeks, weight loss, night sweats, low-grade fever, and hemoptysis in advanced disease. Diagnostic confirmation combines: sputum smear microscopy (rapid, but lower sensitivity), Xpert MTB/RIF or similar molecular assays (result in under two hours, and simultaneously screens for rifampin resistance), chest radiography, and mycobacterial culture (the gold standard for drug-susceptibility testing, though it takes 2–6 weeks to result).

Because culture and full drug-susceptibility testing take weeks, empiric therapy with the standard RIPE regimen begins immediately upon a positive rapid test, and is adjusted later only if resistance is subsequently confirmed.

Directly Observed Dosing — Watching Every Pill Go Down

Directly Observed Therapy (DOT) is the WHO-endorsed cornerstone of the DOTS strategy: a healthcare worker, community volunteer, or trained observer physically watches — in person or increasingly via live video (VOT) — every single dose being swallowed. It converts "the patient says they took it" into a verified, documented fact.

• 1994: WHO DOTS strategy launched (directly observed treatment, short-course)
• ≥95%: Target observed-dose adherence (to reliably achieve cure)
• 2: Observation modes (in-person or video-observed therapy (VOT))
• 7: Doses per week (daily regimen) (no self-administered gaps)

Why observation, not just prescription

TB treatment failure is rarely about drug potency — it is overwhelmingly about consistency. A 6-month, multi-drug course with early symptom relief (patients often feel better within 2–3 weeks) creates strong incentive to stop early or skip doses. Unlike short antibiotic courses, TB therapy must continue long after the patient feels cured, because slow-growing and dormant sub-populations of bacilli are only eliminated by the full duration.

DOT removes ambiguity: each observed dose is logged with a timestamp and observer identity. This creates an auditable adherence record that pill counts or patient self-report cannot match, and lets clinicians intervene the moment a dose is missed rather than discovering non-adherence months later at relapse.

In-person and video-observed therapy

Traditional DOT requires the patient to attend a clinic, pharmacy, or have a health worker visit daily — highly effective but logistically demanding for both patient and health system, especially in resource-limited or rural settings.

Video-observed therapy (VOT) has been validated as non-inferior to in-person DOT in multiple trials: the patient records a short video of themselves taking each dose (often live via a smartphone app), which is reviewed by an observer. VOT dramatically reduces the burden on patients while preserving the core principle — a human confirms the dose was actually swallowed, not merely dispensed.

A missed observation is treated the same as a missed dose. In DOT programs, adherence is measured as the percentage of doses actually witnessed — not the percentage of pills handed out.

Intensive Phase Completion — Sputum Conversion at Eight Weeks

The first eight weeks (2 months) of the four-drug intensive phase are designed to rapidly reduce the enormous actively-dividing bacterial burden. By the end of this phase, most patients become sputum smear-negative — the clearest clinical signal that treatment is working and that the patient is dramatically less infectious to others.

• 8 weeks: Intensive phase length (~56 daily observed doses)
• ~80–90%: Typical smear conversion (of patients smear-negative by month 2)
• extend phase: Persistent smear-positive (triggers extended intensive dosing / resistance work-up)
• sharp: Infectiousness drop (within 2–3 weeks of effective therapy)

What the intensive phase accomplishes

Rifampin and isoniazid rapidly kill the large population of fast-growing extracellular bacilli, driving the bacterial load down by orders of magnitude within days to weeks. Pyrazinamide contributes a distinct, essential job: it kills bacilli hiding inside acidified macrophages and necrotic tissue, populations the other drugs reach poorly. Ethambutol's main role during this window is preventing the amplification of resistant mutants while susceptibility results are pending.

The combined effect is why patients often feel clinically much better within 2–4 weeks — cough eases, fever resolves, energy returns — even though the full course still has months to run.

Sputum monitoring and its meaning

Follow-up sputum smear microscopy at the end of month 2 is a required checkpoint in most national TB programs. A negative conversion confirms the intensive phase achieved its bactericidal goal and clears the patient to move into the continuation phase.

If sputum remains smear-positive at 8 weeks, this can reflect slower response, poor adherence during the intensive phase, or unrecognized drug resistance — any of which may prompt extending the intensive phase, repeating drug-susceptibility testing, or intensifying DOT observation before proceeding.

Missed doses during the intensive phase are especially costly: they allow the largest, fastest-dividing bacterial population to persist and selects preferentially for any pre-existing resistant mutants.

Continuation Phase — Fewer Drugs, Same Discipline

The continuation phase drops from four drugs to two — rifampin and isoniazid — for the remaining four months (about 18 weeks). The bacterial burden is now far smaller, but a persistent, slower-metabolizing sub-population must still be fully sterilized, and that requires the same daily rigor as the intensive phase, not less.

• ~18 weeks: Continuation phase length (4 months, rifampin + isoniazid)
• ~182: Total course dose count (daily doses across 6 months)
• 4 → 2: Drug count reduction (pyrazinamide & ethambutol dropped)
• markedly higher: Relapse risk if interrupted (vs. completed continuation phase)

Why the easier phase is the riskiest for dropout

Paradoxically, the continuation phase is where adherence most often breaks down. Patients feel well, the pill burden has dropped, and the sense of urgency fades — yet this is exactly when the last, hardest-to-kill persister bacilli are being cleared. Rifampin's unique ability to kill slowly-metabolizing "persister" organisms during their brief metabolic bursts is what ultimately sterilizes the lesion; interrupting the daily rhythm gives these survivors a chance to resume replication.

DOT programs frequently see adherence dip in months 3–5 of therapy, which is why sustained observation — not just during the dramatic early weeks — is built into program design.

Consequences of an incomplete continuation phase

Stopping therapy early, or accumulating substantial missed doses, during the continuation phase is strongly associated with clinical relapse — recurrence of active disease, often months after the patient believed they were cured. Relapsed disease is also more likely to have acquired resistance to rifampin and/or isoniazid, since surviving bacilli were exposed to sub-therapeutic, inconsistent drug pressure — exactly the condition under which resistant mutants are selected and amplified.

A regimen taken correctly for 5 months and then abandoned is not "almost a cure" — it recreates the conditions for both relapse and resistance, often requiring a far longer, more toxic retreatment regimen.

Treatment Completion & Outcome — Cure, Relapse, or Resistance

At the close of the 6-month course, the documented DOT adherence record — not just the calendar date — is what determines the outcome. A fully observed course drives cure rates above 95%; an interrupted or poorly adherent course carries meaningfully elevated risk of relapse and of selecting for multidrug-resistant TB (MDR-TB).

• ≥90%: WHO treatment success target (End TB Strategy milestone)
• ~88%: Global treatment success (2022) (drug-susceptible TB cohorts)
• ~95–98%: Cure with sustained ≥95% adherence (directly observed cohorts)
• inconsistent dosing: MDR-TB driver (the single largest modifiable risk factor)

Declaring cure

A patient is classified as "cured" when they complete the full prescribed course with documented adherence and have bacteriologic confirmation (smear or culture negative) at or near the end of treatment, without evidence of failure. "Treatment completed" is used when the course finished on schedule but confirmatory bacteriology wasn't repeated — both count toward treatment success in WHO reporting, and both require the underlying adherence record to be credible.

What happens when adherence falls short

Poor or interrupted adherence produces a spectrum of bad outcomes: treatment failure (persistent positive cultures despite therapy), relapse after apparent cure, loss to follow-up, and — most consequentially for public health — acquired drug resistance. Because each of the four/two drugs has a different resistance mechanism, exposing bacilli to fluctuating, sub-therapeutic drug levels (the hallmark of missed or partial doses) is precisely the selective pressure that breeds multidrug-resistant strains, which then require 9–20 month regimens with more toxic, more expensive second-line drugs and cure rates well below first-line therapy.

The entire rationale for DOT is that curing tuberculosis is not primarily a pharmacology problem — the drugs work — it is an adherence-engineering problem. Every dose witnessed is a resistance event prevented.