Diabetic nephropathy is one of the triad of specific microvascular complications in the eye, kidney and peripheral nerve, recognised as such in the 1950s (Root et al, 1954). The association between diabetic and renal abnormalities was known in the 19th century but it was not until the description of nodular glomerulosclerosis by Kimmelstiel and Wilson (1936) in the 1930s that the pathological basis of nephropathy was established.
Diabetes is the most common single cause of end-stage renal failure worldwide and represents a major public health problem (US Renal Data System, 2008). Early identification and evidence-based intervention are critical to prevent development and to slow progression.
Although the kidneys are generally enlarged mainly due to tubular hyperplasia, the histological appearance at diagnosis of type 1 diabetes is essentially normal. The earliest pathological abnormality is increased thickening of the glomerular capillary basement membrane due to an accumulation of matrix material (Osterby, 1992).
Nearly all people with diabetes will demonstrate this abnormality after 10 years. A minority will show a steady increase in matrix in the areas between the capillaries (the glomerular mesangium), which eventually obliterates them and reduces the filtration capacity of the kidney, ultimately leading to organ failure (Figure 1) (Osterby, 1992). This process takes many years and the pathological features and clinical course are pathognomonic of diabetic nephropathy.
At some stage the capillaries will start to leak proteins (initially albumin, but larger molecules as nephropathy progresses) and these can be detected in the urine. Albuminuria is thus the earliest clinical feature of nephropathy (Marshall and Flyvbjerg, 2006).
As filtration surface is lost secondary to capillary occlusion by matrix material, then glomerular filtration rate gradually declines (at rates of 4–10 mL/min/year) and plasma creatinine and urea concentrations start to rise (Marshall and Flyvbjerg, 2006).
Finally, an important clinical correlate is systemic blood pressure, which rises as albuminuria increases and glomerular filtration declines. High blood pressure accelerates the pathological processes and is an important target for intervention (Marshall and Flyvbjerg, 2006).
The same processes can be seen in type 2 diabetes and the pathological features in the kidney are broadly the same (White and Bilous, 2000). However, because the precise onset of hyperglycaemia is difficult to determine, individuals may have established nephropathy at diagnosis of diabetes. Moreover, many will have pre-existing vascular disease and hypertension, so there may be other causes of renal disease, such as ischaemia, and blood pressure may be high before diabetes develops.
Older people (particularly women) may have recurrent urinary tract infections, which may cause tubulointerstitial damage contributing to functional impairment. Thus, the natural history of kidney disease in people with type 2 diabetes can vary depending on the balance of underlying pathological causes (Fioretto et al, 1996).
Apart from hyperglycaemia and hypertension, there are other processes that are thought to contribute to nephropathy development (Table 1).
Diagnostic tests and staging
Classically, the diagnosis of nephropathy depended upon the detection of proteinuria in a person with diabetes. The development of routine urine testing dipsticks for protein made diagnosis easier but these methods were only sensitive to an albumin concentration of around 300 mg/L.
The development of more sensitive assays for albumin in the 1980s demonstrated that people developing nephropathy had smaller increases in albuminuria long before the routine tests were positive. This phenomenon was termed “microalbuminuria” (not a great term as the albumin is the same but just present in smaller amounts) or “incipient nephropathy”. Traditional dipstick-positive albuminuria then became known as “macroalbuminuria” or overt (sometimes clinical) nephropathy.
A consensus conference defined the limits of normo-, micro- and macroalbuminuria based on timed urine collections (Kidney Disease Outcomes Quality Initiative, 2007; Royal College of Physicians of Edinburgh, 2007). However, these are cumbersome for individuals to collect and labour intensive to analyse, so spot urine samples for albumin corrected for urinary concentration of creatinine (the albumin–creatinine ratio) have been adopted and diagnostic thresholds defined (Table 2).
It must be remembered that albuminuria is a continuous variable, so any cut-off point defining disease is slightly arbitrary and there will be false positive and negative results, particularly at the upper or lower limits of disease or stage classification. The situation is further complicated because microalbuminuria can be found in people with hypertension but without diabetes; in the presence of urinary-tract infections; in people with metabolic syndrome; and in ischaemic nephropathy or tubulointerstitial disease. It is therefore much less specific for nephropathy in type 2 diabetes. A list of the causes of false positive and negative tests is shown in Table 2.
Glomerular filtration rate (GFR)
The detection of albuminuria is the cornerstone of diagnosis of nephropathy. However, of immediate relevance to the patient and clinician is glomerular filtration rate (GFR).
At diagnosis, GFR can be elevated in people with type 1 or 2 diabetes. This is often termed “hyperfiltration” and may contribute to later nephropathy development. The rate of decline thereafter determines the progression of nephropathy and likely timing of end-stage renal disease (ESRD) requiring renal replacement therapy. As it is important to plan this well in advance, then an estimate of GFR is clinically important. Precise estimates of GFR can be performed using infusions of neutral molecules, such as inulin, and measuring their appearance in the urine (Stevens et al, 2006).
Calculation of clearance for a given period of time will derive GFR:
GFR = u.v/p
(where u = urine concentration of marker; v = urine volume per unit time; and
p = plasma concentration of marker).
Infusion of filtration markers is clearly of limited routine utility. Endogenous creatinine, however, can serve almost as well. Creatinine is produced from muscle cells as part of normal metabolism and is completely filtered by the renal glomerulus. Under study conditions, its production and excretion are in balance and it can be used as a filtration marker. A timed (usually 24 hour) urine collection can thus derive an estimate of GFR from creatinine clearance using the above formula. This estimate, however, is still dependent on a urine collection (Stevens et al, 2006).
As GFR declines, plasma creatinine concentrations will rise, but do not cross the upper limit of normal until there is significant loss of filtration capacity. In 1999, researchers used the patient database from the Modification of Diet in Renal Disease (MDRD) study to derive an equation that would convert a plasma creatinine concentration into an estimate of GFR (now called eGFR) (Levey et al, 1999; 2009). An alternative method called the Cockroft-Gault equation also exists but this estimates creatinine clearance, not GFR, and requires a measure of body weight.
The modified MDRD equation:
eGFR = 175 × (serum creatinine [µmol/L] × 0.0113)−1.154 × age (years)−0.203
(Multiply by 0.742 if female, multiply by 1.21 if of Afro-Caribbean origin).
This estimated GFR has been used as a basis for diagnosis and staging of chronic kidney disease (CKD) (Levey et al, 1999). This classification, however, does not map well to that based on albuminuria and is therefore not specific to diabetic nephropathy (Table 3).
Creatinine has its limitations as a marker of filtration and this must be borne in mind when interpreting eGFR (Table 4). Moreover, eGFR tends to underestimate true GFR, particularly at values above 60 mL/min/1.73 m2. However, eGFR is an important way of recognising impairment of renal function at low serum creatinine concentration.
In addition, large intervention trials in people with cardiovascular (CV) disease have shown that a reduced eGFR is an independent risk for morbidity and mortality and this relationship is also true for people with diabetes (Anavekar et al, 2004; Go et al, 2004). In South Tees, mortality rates were twice as high in people with diabetes and an eGFR of <30 mL/min/1.73 m2 compared with those with a value of >90 mL/min/1.73 m2 (Nag et al, 2007). Thus, the detection of a falling eGFR should prompt rigorous management of CV disease risk factors.
Incidence and prevalence of nephropathy depends on the diagnostic criteria and the population under study. Using albuminuria, reported transition rates from normo- to microalbuminuria are around 1–2% per annum and are about the same for type 1 and type 2 diabetes (Adler et al, 2003). However, these rates can be strongly influenced by other factors, such as duration of diabetes, ethnicity and presence of hypertension, CV disease or obesity. Transition rates from micro- to macroalbuminuria are slightly higher at approximately 3% per annum, but this is heavily influenced by the baseline albuminuria – the higher this is, the greater the rate of transition (ACE Inhibitors in Diabetic Nephropathy Trialist Group, 2001).
Prevalence rates are much more variable and dependent on the population under study. In general, population-based studies (not confined to secondary care) report rates for microalbuminuria of 12–27% and 19–42% for type 1 and type 2 diabetes, respectively. For macroalbuminuria the reported range is even wider at 0.3–24% for type 1 and 9–33% for type 2 diabetes (Bilous, 1996).
End-stage renal failure is easier to define but rates are not linear with duration. Using a national disease register, rates of 2.2% and 7.8% for people with type 1 diabetes with 20 and 30 years’ duration, respectively, have been reported from Finland (Finne et al, 2005). For the UKPDS (UK Prospective Diabetes Study) cohort of people with newly diagnosed type 2 diabetes, 0.6% of people required renal-replacement therapy or died from renal failure after 10.4 years of known diabetes duration (Adler et al, 2003; Bilous, 2008).
The main reason for the discrepancy in rates of ESRD between type 1 and 2 diabetes is the increased CV mortality seen in people with nephropathy generally, and those with a reduced eGFR specifically. In the UKPDS, mortality was 2- to 3-fold greater in those with micro- or macroalbuminuria compared with normoalbuminuria. For those with a plasma creatinine >175 µmol/L or requiring renal replacement therapy, mortality was 14-fold greater (Adler et al, 2003). Thus, many people with nephropathy are dying before entering ESRD requiring renal replacement therapy.
Encouragingly, recent data from the USA has suggested that rates of ESRD requiring renal replacement therapy have been declining since 1996 at around 3.4%/year/100000 people with diabetes. The reasons are unclear but probably reflect better overall diabetes and blood pressure management (Burrows et al, 2010).
There are no specific clinical features of nephropathy in its early stages. In people with type 1 diabetes a rise in blood pressure is a subtle sign but usually accompanies an increase in albuminuria (Marshall and Flyvbjerg, 2006).
The clinical features of established nephropathy are often dictated by concomitant comorbidities that can be diabetes specific (retinopathy and neuropathy) or due to macrovascular disease in the coronary, cerebral or peripheral vasculatures. The majority of people entering end-stage renal failure due to diabetic nephropathy will have evidence of some or all of these complications.
In only a minority of people does the proteinuria become so great as to lead to the nephrotic syndrome of hypoalbuminaemia, peripheral oedema, hypercholesterolaemia and heavy proteinuria. Such people have a poor prognosis from the cardiorenal perspective.
As renal impairment gets worse, anaemia due to erythropoietin deficiency is more common and is said to occur earlier in people with diabetic nephropathy compared with those with non-diabetic kidney disease for any given GFR (Bosman et al, 2001). Prevalence studies suggest that around 15% of people with diabetes will have a World Health Organization-defined anaemia (<12 g/dL in premenopausal women; <13 g/dL for men) and these rates increase as GFR declines (Jones et al, 2010).
Hyperphosphataemia, hypocalcaemia and secondary hyperparathyroidism are also features of declining GFR and can lead to osteodystrophy and possibly contribute to macrovascular calcification.
As GFR declines towards CKD stage 5, symptoms of uraemia such as nausea, anorexia, pruritus, bad taste, tiredness and weight loss (sometimes masked by increasing peripheral oedema) develop. The occurrence of these is a sign that renal replacement therapy is imminent.
Management in primary care and when to refer
Tight glycaemic control is the only therapy shown to prevent development of microalbuminuria in type 1 diabetes. The DCCT (Diabetes Control and Complications Trial) showed an approximately 50% reduction in microalbuminuria after 9 years of tight control. This benefit continued for 8 years after the study completed despite the fact that HbA1c levels were similar in the original intensively and conventionally treated cohorts during follow-up. Even after this duration of study, there was no significant impact on the numbers needing renal replacement therapy partly because there were so few events (DCCT/Epidemiology of Diabetes Interventions and Complications [EDIC] Research Group, 2003).
For people with type 2 diabetes, the UKPDS showed a smaller but still significant reduction in incident microalbuminuria in the intensively treated group. In addition, although the numbers were very small, fewer people had a doubling of their baseline serum creatinine (roughly equivalent to a halving of GFR) in the intensive arm (UKPDS Group, 1998a). Current guidance suggests a target HbA1c level of <7.5% (<58 mmol/mol) in people with type 1 diabetes and 6.5% (48 mmol/mol) in those with type 2 diabetes to prevent microvascular complications (National Collaborating Centre for Chronic Conditions [NCCCC], 2004; NICE, 2009). There is no conclusive evidence of an effect of tight glycaemic control on nephropathy development once micro- or macroalbuminuria has developed. NICE (2009) recommendations for kidney damage in type 2 diabetes are outlined in Table 5.
Once micro- or macroalbuminuria has developed, blood pressure management is critical. All patients should be given general advice about reducing dietary salt and alcohol, weight reduction and increasing exercise. However, most will also require drug therapy.
Drugs that block the renin–angiotensin system (RAS) have not been shown to prevent microalbuminuria in people with type 1 or type 2 diabetes who have well controlled blood pressure and who are at low overall CV risk (Bilous et al, 2009). For hypertensive people, or those who have already had a CV event, then angiotensin-converting enzyme (ACE) inhibitors have been shown to prevent the development of microalbuminuria (Heart Outcomes Prevention Evaluation Study Investigators, 2000).
Once people have persistent microalbuminuria then ACE inhibitors in type 1 diabetes and angiotensin receptor blockers (ARBs) in type 2 diabetes reduce progression to macroalbuminuria and increase regression to normoalbuminuria over and above their blood pressure-lowering effect (ACE Inhibitors in Diabetic Nephropathy Trialist Group, 2001; Parving et al, 2001). However, none of these studies were powered to detect any impact on rates of ESRD development. There is, however, good evidence of benefit of ACE inhibitor therapy once people with type 1 diabetes have macroalbuminuria and a reduced GFR (Lewis et al, 1993). In those with type 2 diabetes the effect is smaller but still significant and has only been conclusively established for ARBs (Brenner et al, 2001; Lewis et al, 2001).
The UKPDS showed that many people with type 2 diabetes require three or more drugs to control their blood pressure to target, so although RAS blockade forms the cornerstone of therapy, other agents will almost certainly need to be added (UKPDS Group, 1998b).
A high dietary salt intake will reduce the effectiveness of RAS blockers so reduction should be reiterated for all people taking them. Diuretics work synergistically with RAS-blocking agents. For people with CKD stage 3 or worse then loop diuretics rather than thiazides are indicated. Calcium channel blockers are the next agent recommended in the British Hypertension Society guidelines, but beta-blockers are also useful in people with a history of ischaemic heart disease (Williams et al, 2004). Concerns about their use in people with hypoglycaemia unawareness are probably overstated, although it is prudent to use cardioselective agents.
Current hypertension guidance suggests a blood pressure target of <130/80 mmHg (<125/75 mmHg if proteinuria is >1 g/day) (Williams et al, 2004; NCCCC, 2006). In practice, this can be difficult to achieve without polypharmacy to a degree that has intolerable side-effects or poses a problem for concordance and compliance. However, any reduction in blood pressure is of benefit, so it is critical to negotiate acceptable targets with people on an individual basis. A Cochrane review has found a reduction in dietary protein to be beneficial in terms of slowing nephropathy progression (Robertson et al, 2007).
As most people with diabetes and nephropathy have macrovascular disease, a small minority will have a functional renal artery stenosis. Renal blood flow in these people is dependent on a functioning RAS so inhibition using ACE inhibitors or ARBs can result in an acute deterioration of renal function. For this reason, it is recommended that serum creatinine and potassium are checked within 2 weeks of initiation of RAS blockade and after any increase in dose (Williams et al, 2004; NICE, 2009). A rise in serum creatinine of >75 µmol/L should raise the possibility of renal artery stenosis. Increases less than this are common and not usually of clinical significance.
Because people with nephropathy have an increased risk of CV disease, cholesterol-lowering therapy and low-dose aspirin should be considered for those who have a 5 year risk >20% based upon the Framingham equation (Joint British Societies, 2005). In reality, most people with diabetes will have evidence of pre-existing macrovascular complications and should be prescribed such therapy for secondary prevention anyway.
People with nephropathy are at high risk of foot ulceration and many will have established retinopathy. It is important that they continue to access foot and retinal screening.
The Steno-2 Study (Gaede et al, 2008) of multifactorial CV risk intervention in people with type 2 diabetes with microalbuminuria at baseline demonstrated long-term benefits on mortality, development of nephropathy and ESRD, as well as CV complications, including myocardial infarction and amputation. The treatment was for RAS-blocking drugs in all participants in the intensively treated group, lipid-lowering therapy with a target total cholesterol <4.5 mmol/L, intensive glycaemic control with a target HbA1c level of <6.5% (<48 mmol/mol), low-dose aspirin, antioxidants (vitamins C and E) and lifestyle changes, including stopping smoking, weight reduction and increasing exercise. As with the DCCT/EDIC and UKPDS these benefits continued beyond the end of the trial.
NICE has issued guidance for the management of both type 1 and type 2 diabetes that includes advice on diabetic nephropathy (National Collaborating Centre for Chronic Conditions, 2004; NICE, 2009). Moreover, there is guidance for CKD generally, which also includes a section on diabetes (Joint Specialty Committee on Renal Medicine of the Royal College of Physicians and the Renal Association, Royal College of General Practitioners, 2006). The Joint British Societies (2005) have published guidance on CV risk factor management.
When to refer?
The Royal College of General Practitioners has issued guidance on the indications for referral for people with CKD (Joint Specialty Committee on Renal Medicine, 2006; Table 6). People with chronic, stable renal impairment and well-controlled glycaemia and blood pressure probably do not need referral even if they are at CKD stage 4. However, any person in whom glycaemia or blood pressure control is proving difficult and/or has a rapidly declining GFR of >5 mL/min/year or >10 mL/min/10 years should be referred as they are at risk of requiring renal replacement therapy and this needs to be planned early. Similarly those with anaemia or calcium and phosphate problems should also be referred.
People with type 2 diabetes can also develop renal disease other than nephropathy and should be referred if they show any of the unusual features outlined in Table 6. The presence of retinopathy in a person with diabetes and albuminuria makes a diagnosis of diabetic nephropathy almost certain.
For many people with diabetes the diagnosis of nephropathy and possible renal failure is an ominous one. Most will be aware of the implications and will show a classic bereavement reaction similar to that seen following a diagnosis of cancer or heart disease. For these reasons it is important to prepare people and their partners, carers and families well in advance of the need for renal replacement therapy. Many units offer pre-end-stage education and counselling as part of preparation for dialysis. For younger people, live donor kidney transplantation may be an option and requires careful and sensitive management.
Boxes 1 and 2 provide two case studies highlighting some of the practical issues related to the management of people with diabetic nephropathy.
Nephropathy is a serious complication of diabetes and is associated with significant mortality and comorbidity. However, there is a strong evidence base for therapies that can prevent development and slow its progression.
Thirty years ago the median time from development of macroalbuminuria to ESRD was just 7 years (Watkins et al, 1977) – it is now closer to 20. Moreover, the numbers of people requiring renal replacement therapy appear to be falling, at least in the USA. This is probably the result of better overall care in terms of glycaemia and blood pressure and CV risk factor management. The remaining challenge is to try and prevent people developing nephropathy in the first place.