Final answer:
The client should take the morning dose and have the blood drawn 2 hours after taking the dose.
Step-by-step explanation:
To prepare for the blood test to measure the serum concentration of cycloserine (Seromycin Pulvules), the client with tuberculosis should follow the guidance of taking the morning dose and having the blood drawn 2 hours after taking the dose. This ensures that the medication has reached its peak concentration in the bloodstream. Withholding the morning dose or doubling the dose before the test may affect the accuracy of the test results.