SECTION 3:

Tests used in the assessment of kidney function

Both blood and urine tests can be used to assess kidney function.

Glomerular filtration rate (GFR)

Exact and direct measurement of GFR is complicated so laboratories estimate the GFR based on a range of measurements, including serum creatinine level, age, sex and ethnicity.1 The estimation of glomerular filtration rate (eGFR) can be affected by factors such as extremes of muscle mass, sex and age of the person being tested. Dietary intake, especially meat, can also affect the results.2 Furthermore, changes in creatinine levels may not be seen until significant levels of renal impairment have occurred, making it a later marker of renal impairment compared with other tests.

The eGFR is considered to be normal if it is over 90ml/min/1.73m2 and eGFR of 60-89ml/min/1.73m2 indicates mild reduction in kidney function relative to the normal range for a young adult.2 If the eGFR is found to be less than 60ml/min/1.73m2, NICE recommends that a repeat test should be carried out within 2 weeks in case this is the first sign of impending acute kidney injury (AKI).2 If there is no evidence of further accelerated loss of eGFR, the test should be repeated after another 3 months. In patients with a very high or very low muscle mass, eGFR should be interpreted with caution as GFR will be overestimated in people with reduced muscle mass, whereas an increased muscle mass will lead to underestimation of the GFR.2

Creatinine clearance (CrCl)

An alternative, and potentially more accurate way of assessing glomerular function via a blood test is to determine the level of creatinine clearance (CrCl). This can be calculated using the Cockcroft-Gault formula, which can be found on most practice templates or through an online calculator, available at https://www.mdcalc.com/creatinine-clearance-cockcroft-gault-equation. Although the values used in eGFR and CrCl measurements appear very similar in terms of the numbers used, they are not equivalent. In cases of abnormally low or high BMI (<18kg/m2 or >40kg/m2) creatinine clearance is the preferred measurement to assess renal function.3

Urine testing

In healthy people, very little or no protein is present in the urine.4 Excessive protein excretion in the urine (proteinuria) is a significant factor for both kidney disease and for cardiovascular morbidity and mortality.4 The level of protein is assessed by a urine test to measure the amount of albumin (protein) that has been excreted in urine, and the levels of creatinine present; the albumin/creatinine ratio can be calculated and used to determine kidney function. ACR is more sensitive for detecting low levels of proteinuria and is the recommended method for screening in patients with diabetes. NICE does not recommend the use of reagent strips unless they can specifically measure low albumin concentrations and express the results as an ACR.2

An ACR of between 3 mg/mmol and 70mg/mmol should be confirmed by a subsequent early morning sample.2 A positive ACR (3mg/mmol or more) on two or more occasions is enough to confirm DKD.2

Measuring albuminuria may identify diabetic kidney disease earlier than estimating GFR. By the time eGFR falls to below 60 ml/min/1.73m2, 60-80% of nephron mass – an indicator of renal functional reserve – may have been lost (Figure 2).5

URINARY ALBUMIN EXCRETION (UAE) IS AN EARLY INDICATOR OF DIABETIC KIDNEY DISEASE.5

Tonneijck graph

Adapted from Tonneijck, et al. 2017

Early detection using microalbuminuria screening could help to improve renal outcomes, by allowing for earlier intervention, but National Diabetes Audit figures for 2017-18, show 34% of people with type 2 diabetes did not have a urine albumin test – a significant drop over the last few years.6

The National Chronic Kidney Disease Audit emphasises the importance of performing both blood tests to measure creatinine levels, and urine tests to detect albumin.7 Both tests are needed to accurately diagnose DKD.7 On average, 86% of people with diabetes have annual blood tests for eGFR but only 54% have the relevant annual urine tests.7 While over 80% of patients with confirmed CKD had an eGFR test in the previous year, only 31% had a repeat ACR test,7 required to monitor progression.

Although figures from the national audits for diabetes and CKD differ, both indicate that ACR testing is not performed in line with guideline recommendations.6,7

Care should be taken to repeat any abnormal tests in order to ensure that any change is not transitory, and to identify the presence of acute kidney injury (AKI), which will require more urgent intervention. An abnormal eGFR should be repeated twice, within 2 weeks, to exclude accelerated loss, i.e. AKI, and three times over not less than 90 days to identify the rate of progression of renal impairment.2

Job code: UK/DIA-19020d DOP: June 2020

References

  1. The Renal Association. About eGFR; 2018 https://renal.org/information-resources/the-uk-eckd-guide/about-egfr/ [Accessed June 2020]
  2. NICE CG182. Chronic kidney disease in adults: assessment and management; July 2014 (Updated January 2015). https://www.nice.org.uk/guidance/cg182 [Accessed June 2020]
  3. Medicines and Healthcare products Regulatory Authority. Prescribing medicines in renal impairment: using the appropriate estimate of renal function to avoid the risk of adverse drug reactions; October 2019. https://www.gov.uk/drug-safety-update/prescribing-medicines-in-renal-impairment-using-the-appropriate-estimate-of-renal-function-to-avoid-the-risk-of-adverse-drug-reactions [Accessed June 2020]
  4. The Renal Association. Proteinuria https://renal.org/information-resources/the-uk-eckd-guide/proteinuria/ [Accessed June 2020]
  5. Tonneijck L, et al. J am Soc Nephrol 2017;28(4):1023-39
  6. NHS Digital. National Diabetes Audit, 2017-18. Report 1: Care processes and treatment targets; 2019. https://files.digital.nhs.uk/88/F1E544/National%20Diabetes%20Audit%202017-18%20Full%20Report%201%2C%20Care%20Processes%20and%20Treatment%20Targets.pdf [Accessed June 2020]
  7. National Chronic Kidney Disease Audit. National Report (Part 1); January 2017. https://www.hqip.org.uk/wp-content/uploads/2018/02/Rc8QAw.pdf [Accessed June 2020]