Final answer:
The medication is effective if the cultures of sputum yield a negative result.
Step-by-step explanation:
The nurse determines that the medication is effective if the cultures of sputum yield a negative result after the client has been taking pyrazinamide for 6 months. Since pyrazinamide is primarily used to treat Tuberculosis (TB), the negative sputum cultures indicate that the client's TB infection is being successfully treated.