Low Estimated GFR and Normal Creatinine

Introduction

Serum creatinine is the most widely used test to assess kidney function. It is well known among kidney doctors that somebody can have a “normal” creatinine and decreased kidney function. This has to do with the fact that the serum creatinine depends not only on kidney function but also on how much creatinine is being made (which depends a lot on body composition). A more accurate estimate of kidney function, the estimated GFR, is being incorporated in standard lab reports. Not infrequently, people are learning that they have decreased kidney function from their lab reports but their doctors are telling them that their creatinine is “normal”. This leads to a lot of confusion and many wonder if their eGFR is accurate.

Serum Creatinine

Serum creatinine is the most widely used test to assess kidney function. Creatinine is created from creatine, a compound found almost exclusively in muscle and made at a relatively constant rate. Creatinine leaves the muscle and enters the blood, where it is then removed by the kidneys. If the kidneys lose their ability to filter blood, more creatinine will accumulate and the serum creatinine will rise.

Glomerular Filtration Rate

The glomerular filtration rate (GFR) is a more accurate estimate of the filtering capacity of the kidneys. It is usually expressed as milliliters (mL) per minute (min) and adjusted to a “standard” body size with a surface area of 1.73 meters². The normal GFR ranges between 95 -120 mL/min/1.73m² but it varies depending on age, gender and body size. GFR is usually estimated (eGFR) from a mathematical equation based principally on the serum creatinine.

eGFR Has Limitations

Although the estimated GFR is often a more accurate estimate of kidney function than the serum creatinine alone, it has its limitations. The formula used to calculate it is very sensitive to the serum creatinine and can be dramatically affected by lab errors when the creatinine is relatively normal. Also, it assumes a certain body type based on the persons age, gender and race but this may not be true. For example, if you are a very tiny woman with very little muscles, you may be producing a lot less creatinine than the average woman with a similar age and racial background.

Normal Creatinine but Low eGFR

If you were told that you have a low GFR despite a “normal” creatinine, the odds are that you have decreased kidney function but it is unlikely to be severe. Whether you have a mild decrease (< 90 mL/min/1.73m²) or a moderate decrease (< 60 mL/min/1.73m²) is less certain. At these levels, the eGFR is very sensitive to the accuracy of the serum creatinine (see table below). Under these conditions, it is prudent to repeat the serum creatinine periodically and screen for obvious kidney disease. If there is evidence of kidney disease or a worsening of the serum creatinine or eGFR, a referral to a kidney doctor would be prudent.