A 12-year-old boy, average student of class VII, presented with 1 year history of generalized tonic clinic seizures which lasted for 2-3 min. In the initial 2-3 days of his illness, he had 2-3 seizures daily for which he received carbamazepine 200 mg twice daily. Seizure though was controlled, but he started having carpopedal spasms lasting for 1-2 min. His elder brother aged 16 years also had similar illness. Examination revealed that the patient was of average built and nutrition (height 157.5 cm, weight 48kg) and there were no dysmorphic features. The neurological examination was normal except Trousseau’s sign.
The laboratory studies revealed normal hemoglobin and blood counts. His serum calcium was 6 (ionized 4) (normal 8.5-10.8, ionic 4.6-5.3) mg/dl, phosphorus 8 (normal 2.5-4.5) mg/dl, alkaline phosphatase 999 U/l, and magnesium 2.2 mg/dl, blood urea nitrogen 8 mg/dl, and creatinine 0.7 mg/dl. 24-h urinary calcium was 150 (normal 200) mg, phosphorous 0.06 (normal 1) g, and creatinine 1.29 (normal 1-2) g. His calculated TmP/GFR was 2.57 mmol/l (normal 1.15-2.44 mmol/l for 2-15 years age).[] The glomerular filtration rate was 122 ml/min. Serum 25-OH vitamin D was 9.98 (normal 9-47) ng/ml and PTH 335.4 (normal 9-55) pg/ml. His electrocardiogram, electroencephalography, radiograph of hand, feet and pelvis were normal. CT scan revealed bilateral hyperdensity in bilateral caudate, globus pallidus, putamen, and right frontal white matter. Cranial MRI revealed T1 hyperintensity in caudate, globus pallidus, putamen, and dentate nuclei bilaterally. T2 and FLAIR images were normal. He was prescribed calcitrol 0.25 mg twice daily and calcium gluconate 500 mg four times daily without anticonvulsant, on which he was asymptomatic till 1 year follow up.
The formula for computing Cockcroft-Gault is given by CrCl = ((140 - age) * adjusted weight * (gender_coefficient)) / (serum creatinine * 72), where the gender_coefficient is 1 if male, and 0.85 if female. The serum creatinine concentration is in mg/dL.
The patient's gender is male, which means that the gender coefficient is 1.
The patient is 12 years old. 
The concentration of creatinine is 0.7 mg/dL. 
The formula for computing the patient's BMI is (weight)/(height * height), where weight is the patient's weight in kg and height is the patient's height in m.
The patient's height is 157.5 cm, which is 157.5 cm * 1 m / 100 cm = 1.575 m. The patient's weight is 48.0 kg. The patient's bmi is therefore 48.0 kg / (1.575 m * 1.575 m) = 19.35 kg/m^2.The patient's BMI is 19.4, indicating they are normal weight.
Because the patient is normal, we take take minimum of the ideal body weight and the patient's body as the patient's adjusted weight for the Cockroft-Gault Equation. Hence, the adjusted body weight is the minimum of the two giving us an adjusted body weight of 48.0 kg.

Using the Cockcroft-Gault equation:
CrCl = ((140 - age) * adjusted weight * gender_coefficient) / (serum creatinine * 72).
Plugging the patient's values gives us ((140 - 12) * 48.0 * 1) / (0.7 * 72) = 121.905 mL/min. Hence, the patient's creatinine clearance is 121.905 mL/min.