Introduction
Serum creatinine is the most widely used test to assess kidney function. The kidneys remove creatinine from the blood and concentrate it in the urine. If the kidneys are failing, serum or plasma levels will rise.
Indirect Marker of GFR
Creatinine is created from creatine, a compound found almost exclusively in muscle, at a relatively constant rate. It leaves the muscle and enters the blood, where it is subsequently removed by the kidneys. Most of the creatinine enters the urine after being filtered by the glomeruli (some is secreted) and the remaining amount accumulates in the serum or plasma.If the kidneys lose their ability to filter blood (GFR decreases), more creatinine will accumulate and serum or plasma creatinine will rise. As a result, creainine is an indirect marker of glomerular filtration rate (GFR) or the functional capacity of the kidneys.
Normal Values
There is no single normal value for serum creatinine because it depends on how much muscle you have. It is often said that a normal value is 0.8-1.3 mg/dL for men and 0.6 to 1.0 mg/dL for women. However, this isn’t always true. For example, a creatinine of 1.0 mg/dL might be considered normal unless it recently increased from 0.6 to 1.0 mg/dL. In this case, it would be very abnormal. As a result, to decide if a creatinine is normal, it is important to take into account the amount of muscle you have and any previous lab results.
Identifying Early Disease
There isn’t a good correlation between serum creatinine and kidney damage early in a disease process. This is because the healthy parts of the kidneys can make up for the damaged parts and so the GFR doesn’t reflect the damage. As a result, by the time the serum creatinine starts to noticeably rise, there is already significant damage.
Monitoring of Disease Progression
Many people believe that once moderately advanced kidney disease is established (typically a creatinine greater than 2.5 mg/dL), you can plot the inverse of creatinine (1/Cr) over time and get a straight line. Assuming that you are losing kidney function at a constant rate, you can extend the line out in time to get a rough idea when the kidneys will completely fail. Although this plot isn’t perfect (many people do not progress at a constant rate), it may be helpful in giving you a rough idea when you might need dialysis. It also can be helpful in recognizing when there is an acute drop in kidney function in addition to the chronic loss.