How accurate is a ct scan?

Difficult to say: A CT scan is a mathematical model of the inside of whatever body part is being scanned, based on the measurement of attenuation of the x-ray beam from multiple points on the outside. The accuracy is determined by whether a specific size abnormality can be seen, which varies with different diseases. A CT abnormality seen is accurate, but the disease causing this may not be easily determined.

It depends on: the potential abnormality being assessed, assuming an appropriate and technically adequate CT scan was performed.