E.l is a 35-year-old 65kg white male being treated for acute mania .he is receiving 300mg lithium carbonate 3 time daily. His serum creatinine concentration is 0.9mg/dl. Calculate the expected lithium concentration just before the morning dose on the forth day of therapy.
Question
E.l is a 35-year-old 65kg white male being treated for acute mania .he is receiving 300mg lithium carbonate 3 time daily. His serum creatinine concentration is 0.9mg/dl. Calculate the expected lithium concentration just before the morning dose on the forth day of therapy.
Solution
To calculate the expected lithium concentration, we need to use the following formula:
Css = (F * Dose) / (Cl * Tau)
Where:
- Css is the steady-state plasma concentration
- F is the bioavailability of the drug (for lithium, this is typically assumed to be 1, as it is well absorbed)
- Dose is the amount of drug administered per dosing interval
- Cl is the clearance of the drug
- Tau is the dosing interval
First, we need to calculate the clearance (Cl). Lithium is primarily excreted by the kidneys, and its clearance can be estimated using the following formula:
Cl (L/h) = (1.5 * CrCl) / Weight
Where:
- CrCl is the creatinine clearance, which can be estimated using the Cockcroft-Gault equation: (140 - age) * weight (kg) / (72 * serum creatinine (mg/dL)) * (0.85 if female)
- Weight is the patient's weight in kg
In this case, the patient is a 35-year-old male weighing 65kg with a serum creatinine concentration of 0.9mg/dL. So, his creatinine clearance (CrCl) would be:
CrCl = (140 - 35) * 65 / (72 * 0.9) = 98.61 mL/min
Then, we can calculate the clearance (Cl) of lithium:
Cl = (1.5 * 98.61) / 65 = 2.28 L/h
The patient is receiving 300mg of lithium carbonate three times daily, so the total daily dose is 900mg. The molar mass of lithium is approximately 6.94g/mol, so this dose is equivalent to 900 / 6.94 = 129.69 mmol.
Therefore, the expected lithium concentration just before the morning dose on the fourth day of therapy (Css) would be:
Css = (1 * 129.69) / (2.28 * 24) = 2.38 mmol/L
Please note that this is a simplified calculation and actual patient results may vary due to a variety of factors. Always consult with a healthcare professional for accurate information.
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