Renal Functional Reserve: Time for a Kidney stress test in clinical practice?

GFR varies under normal physiological conditions and during illnesses. A popular example is a low GFR in vegetarians and a higher GFR in consumers of large quantities of animal protein, even when they have a similar normal renal mass. It is not clear what the maximum GFR can be, but it can be approached with an acute animal protein load. One to two hours after an animal protein load, individuals with healthy kidneys will show a significant rise in their GFR independent of their baseline GFR. The difference between baseline and maximal (i.e. stress or peak) GFR is called the Renal Functional Reserve (RFR).

The maximum capacity of a functioning renal mass is not reflected by the baseline GFR of a given individual. Bellomo et al used an example of 4 different patients to explain this concept. Patients A (animal protein consumer) and B (vegetarian) have the same renal mass but different baseline GFRs owing to different basal protein in-takes levels. Patient A has a GFR of 120 ml/min that can be stimulated to 170 ml/min. Patient B has a baseline GFR of 65 ml/min that also can be stimulated to 170 ml/min. Therefore, the RFR in these two patients is different because they are using their GFR capacity at a different level. Patient C had a unilateral nephrectomy. His baseline GFR corresponds to his maximal GFR under unrestricted dietary conditions. If a moderate protein restriction is applied to his diet, his baseline GFR may decrease and some degree of RFR become evident. Patient D, who is a vegetarian who underwent unilateral nephrectomy, will have a lower baseline compared to patient C but a higher RFR. Therefore, in general, restoring some RFR requires a severe protein restriction, and hence baseline GFR does not always correspond to the extent of functioning renal mass unless we place it in the context of maximal capacity. Bellomo et al concluded the section about GFR by using a very interesting, possibly true, statement:

“In this regard GFR is not unlike a resting ECG for the kidney. When it is grossly abnormal, renal function is impaired, but when it is normal, a stress test is required.”

The GFR rises considerably during pregnancy. This physiological rise is multifactorial and is mainly attributed to increase in cardiac output and renal blood flow. It becomes apparent from the 1st month and peaks at 40% – 50% above baseline levels by the 4th or 5th month of pregnancy. This increase in GFR is referred to as renal hyperfiltration. The RFR is consumed as a part of adaptation to this physiological demand that occur during pregnancy. This was demonstrated by Ronco et al. They assessed GFR changes in pregnant women, with normal kidneys, before and after protein load. After acute protein load, all women had a significant increase in GFR. This rise was more in the first than in the last trimester. This finding explains, at least partially, renal hyperfiltration in pregnancy.

RFR allow for an increase in GFR during stressful conditions to ensure maintenance of adequate kidney function. When RFR is lost or fully utilized and the kidney insult continues, changes in baseline GFR and serum creatinine occur. After an AKI episode, creatinine and GFR may return to normal, displaying an apparent complete recovery of the kidney. Unfortunately, this recovery might be at the expense of reduction or loss of the RFR. In my opinion, without performing a kidney stress test to assess the RFR post-AKI, it will remain unclear whether the recovery from AKI was complete or was it just a biochemical recovery (reflected by creatinine level) at the expense of RFR utilization. Conceptually, recovering baseline GFR and creatinine level post-AKI without recovering the RFR should be labelled as new-onset CKD because it actually reflects an irreversible loss of nephrons/RFR. I have no evidence to support this, but I would hypothesize that these patients who lose their RFR post-AKI are the ones who were shown to progress to CKD in previous studies.

I think the following are potential benefits for using a kidney stress test/ checking RFR:

1. Assessment of recovery post-AKI: It will help to detect patients who are likely to progress to CKD.
2. Assessment of living kidney donors prior to donation: It is likely that a low RFR might increase the long-term risk of CKD during the post-donation period.
3. To assess the risk of AKI in patients undergoing contrast studies and high-risk surgeries.

Of course robust studies are needed to assess the diagnostic and the prognostic utility of RFR and kidney stress test in the clinical setting.


Post by Mohammed Kaballo

Sweet’s syndrome and the Kidney

This is an interesting case which I have been managing over the last six months. A 50-year-old male, with no previous medical illnesses, presented with fatigue, weight loss and arthralgia for several weeks. Clinical examination was unremarkable. Investigations revealed a rise in Creatinine from a baseline of 86 to 120 µmol/l (eGFR= 70 down from 90). His urine dipstick revealed protein (1+) and blood (1+). He had a normal Chest X-ray, but his p-ANCA was positive with a high MPO titre. Renal imaging was normal and he underwent a kidney biopsy which showed non-specific findings i.e. some tubulointerstitial inflammation and glomerulosclerosis with mild features of thrombotic microangiopathy (TMA). There were NO crescents and both immunofluorescence and electron microscopy showed NO immune deposits. Putting it all together, a diagnosis of vasculitis was made and treatment with Steroids & Rituximab was initiated. ANCA titre started to fall and urine sediments disappeared. Unfortunately, he developed a steroid-induced psychosis and had a failed suicide attempt. Subsequently, corticosteroids were discontinued and he was commenced on Azathioprine.

Four months later he presented with fever, constitutional symptoms and skin rash in the form of reddish papules & nodules involving the trunk, neck & face. Lesions got progressively worse and coalesced to form plaques. A skin biopsy confirmed the presence of neutrophilic dermatosis and a diagnosis of Sweet’s syndrome was made. Interestingly there was no evidence of vasculitis in the skin biopsy. He was treated with high-dose systemic steroids and improved dramatically. This was done in a closely monitored environment in-hospital, given his history of steroid-induced psychosis. Currently, he is being thoroughly investigated for any possible underlying malignancies.

What is Sweet’s syndrome?

Sweet’s syndrome, also known as acute febrile neutrophilic dermatosis, is a reactive skin disorder characterized by the sudden onset of papules and nodules which are tender and reddish/purple in colour. These lesions coalesce later to form plaques. It mainly involves the upper extremities, face, or neck and is typically accompanied by pyrexia and peripheral neutrophilia.

It is more common in females and often occurs after a respiratory illness, which is usually mild. More severe disease often occurs with underlying malignancies, drugs or inflammatory conditions e.g. inflammatory bowel disease. Sometimes it could be the first manifestation of the underlying disorder, so whenever it is diagnosed it should prompt further investigation.

Sweet’s syndrome responds dramatically to systemic corticosteroids and may improve or resolve with treatment of the underlying condition. Without treatment, the syndrome may persist for weeks or months but eventually improves, in the majority of cases, without leaving any scars; rarely it may persist and never resolve completely. Recurrences are common.

The diagnosis of Sweet syndrome is based on both clinical and histopathologic findings. A characteristic that distinguishes the lesions of Sweet syndrome from other neutrophilic dermatosis is the absence of vasculitis. However, the presence of vasculitis should not exclude the diagnosis as this may represent an epiphenomenon instead of a primary disease. ANCA have been reported positive in Sweet syndrome and other neutrophilic dermatoses, but this finding is not consistent. This case raised a number of interesting questions:

1-      Was this case a Sweet’s syndrome from the outset rather than an ANCA vasculitis?

ANCAs have been reported to be positive in Sweet syndrome. The kidney biopsy findings, in this patient, were not convincing and the renal course of the disease was not typical of vasculitis. Moreover, it was previously reported that Sweet’s syndrome is associated with renal involvement manifesting most commonly as proteinuria, and less often as haematuria and membranoproliferative glomerulonephritis. Unfortunately, this disease is uncommon and there is a paucity of studies describing the association between Sweet's syndrome and kidney diseases.

2-      Was this case an ANCA vasculitis and the Sweet’s syndrome was a reactive process to the inflammation?

This is another possibility and the association between this syndrome and inflammatory conditions is well known.

3-      Is it a drug-induced Sweet’s syndrome secondary to Azathioprine?

There are several drugs that have been implicated as a cause of this syndrome. Examples are: Azathioprine, Frusemide, Hydralazine, Quinolones and many others.

4-      After complete recover from Sweet’s syndrome and exclusion of underlying malignancy, is it reasonable to continue treating a possible ANCA vasculitis or will it is more appropriate to attribute the positive ANCA and the non-specific renal findings to Sweet’s syndrome per se?

This is a difficult question to answer. The benefits of long-term immunosuppression should be weighed against the risk of infection, other serious adverse effects and the risk of missing an active vasculitis in a young patient.

In conclusion, Sweet’s syndrome is an uncommon disease which could be primary or secondary to an underlying inflammatory process or malignancy. It may cause a positive ANCA test in the absence of active vasculitis. Its association with kidney diseases is not well described and need to be explored.

Post by Mohammed Kaballo

False-positive AKI and the perfectly imperfect biomarker

A small absolute change in serum creatinine level, 0.3 mg/dl, is used by Acute Kidney Injury Network (AKIN) and Kidney Disease Improving Global Outcomes (KDIGO) guidelines to define the presence of Acute Kidney Injury (AKI). The base of this definition was formed by several studies findings of strong association between adverse outcomes and minor changes in serum creatinine level. Subsequently, evidence emerged suggesting that this may not be true to the same extent in people with pre-existing CKD, because variations in serum creatinine concentration are common in these individuals.

As with all other laboratory tests, serum creatinine measurements are affected by within- and between-sample coefficients of variation, intra-individual variation and biologic variation. Biological variation may result from variations in diet, muscle mass and breakdown, tubular secretion, variability in volume homeostasis and from medications uses. The variation in measured serum creatinine level could be as high as 9%. Because only a small increase in serum creatinine is needed to meet AKI criteria, random variation in creatinine level may be a significant contributor to AKI diagnosis in the absence of a true reduction in GFR. This is called a false-positive AKI. It has been shown that high variation in serum creatinine in the period, of days, preceding the development of AKI was not associated with the anticipated inpatient mortality or dialysis. This observation supports the existence of false-positive AKI.

Lin et al demonstrated, using the KDIGO definition, an 8% overall false-positive rate for AKI diagnosis. This rate was much higher, 31%, for the subgroup of CKD patients with serum creatinine ≥1.5 mg/dl. Therefore, an absolute change in serum creatinine of 0.3 mg/dl may represent a relative inconsequential change in GFR in CKD patients rather than a superimposed acute injury.

In my opinion, false-positive AKI could largely explain why most randomized trials for early intervention in AKI have been unsuccessful in improving outcomes. AKI is misclassified under frameworks that do not reflect true GFR reduction. Consequently, patients with false-positive AKI are included in AKI studies and dilute observed effect sizes. This potentially leads to false-conclusions that certain interventions are ineffective and do not improve outcomes. The underlying severe disease is quite likely the actual mediator of adverse outcomes seen in AKI. Therefore, small changes in serum creatinine may be nothing more than a reflection of the severity of the underlying disease process. This point remains a topic of hot debate. Moreover, AKI definition using small increments in serum creatinine level has not been validated among patients with CKD.

It is obvious now that serum creatinine is an imperfect AKI biomarker; especially that it is being used on the basis of a relative change in value of a continuous variable instead of the crossing of a particular threshold. The ideal biomarker would accurately detect true reduction in GFR, be detectable early in the course of renal dysfunction to allow for timely intervention, and predict outcomes. It is likely that current AKI criteria will eventually be modified at least in part by sensitive and specific biomarkers of kidney injury. The use of such biomarkers will help in the development of a new paradigm for classifying AKI that is not only dependent upon serum creatinine. Meanwhile, the awareness about false-positive AKI should be highlighted and the limitations of serum creatinine, as an AKI biomarker, should be re-emphasized.

Authored by Mohammed A. Kaballo, Nephrology Fellow, Ireland