Creatinine - this test is a serum measurement of the level of creatinine in the blood, a waste product produced by the normal breakdown of muscle tissue. This test is useful for assessing and monitoring kidney function and diagnosing kidney disease. Elevated blood creatinine levels can indicate reduced kidney function, suggesting that the kidneys are not effectively filtering waste. This test is often used alongside other kidney function tests to assess the severity of kidney impairment, adjust medication dosages, or monitor the progression of chronic kidney disease.