The reason for maintaining the patient on INH despite the suspicion of MDR-TB is that INH is so potent in treating TB that it is foolish to omit it until there is microbiological proof that it is ineffective. There are also probes available for isoniazid-resistance (katG and mabA-inhA), but these are less widely available. When sensitivities are known and the isolate is confirmed as resistant to both INH and RMP, five drugs should be chosen in the following order (based on known sensitivities):

an aminoglycoside (e.g., amikacin, kanamycin) or polypeptide antibiotic (e.g., capreomycin)
PZA
EMB
a fluoroquinolones: moxifloxacin is preferred (ciprofloxacin should no longer be used;
rifabutin
cycloserine
a thioamide: prothionamide or ethionamide
PAS
a macrolide: e.g., clarithromycin
linezolid
high-dose INH (if low-level resistance)
interferon-γ
thioridazine
meropenem and clavulanic acidDrugs are placed nearer the top of the list because they are more effective and less toxic; drugs are placed nearer the bottom of the list because they are less effective or more toxic, or more difficult to obtain. Resistance to one drug within a class generally means resistance to all drugs within that class, but a notable exception is rifabutin: rifampicin-resistance does not always mean rifabutin-resistance and the laboratory should be asked to test for it. It is only possible to use one drug within each drug class. If it is difficult finding five drugs to treat then the clinician can request that high level INH-resistance be looked for. If the strain has only low level INH-resistance (resistance at 0.2 mg/L INH, but sensitive at 1.0 mg/L INH), then high dose INH can be used as part of the regimen.
In severe chronic kidney disease (GFR<30), the EMB dose should be halved (or avoided altogether). The PZA dose is 20 mg/kg/day (UK recommendation) or three-quarters the normal dose (US recommendation), but not much published evidence is available to support this. When using 2HRZ/4HR in patients on dialysis, the drugs should be given daily during the initial high-intensity phase. In the continuation phase, the drugs should be given at the end of each haemodialysis session and no dose should be taken on non-dialysis days. HIV
In patients with HIV, treatment for the HIV should be delayed until TB treatment is completed, if possible. The current UK guidance (provided by the British HIV Association) is

CD4 count over 200—delay treatment until the six months of TB treatment are complete. CD4 count 100 to 200—delay treatment until the initial two-month intensive phase of therapy is complete
CD4 count less than 100—the situation is unclear and patients should be enrolled in clinical trials examining this question. There is evidence that if these patients are managed by a specialist in both TB and HIV then outcomes are not compromised for either disease.If HIV treatment has to be started while a patient is still on TB treatment, then the advice of a specialist HIV pharmacist should be sought. In general, there is no significant interactions with the NRTIs. Nevirapine should not be used with rifampicin. Efavirenz may be used, but dose used depends on the patients weight (600 mg daily if weight less than 50 kg; 800 mg daily if weight greater than 50 kg).