INTRODUCTIÓN

Despite the significant transplant activity in Spain,1 no consensus has been reached yet in our country about the clinical situations in which a diagnostic biopsy would be recommended, or about how use of protocol or donor biopsies may be improved in clinical practice. Thus, an overall view detects a high heterogeneity between the different centres and even certain practices that may be considered inadequate. For instance, despite the significant
evidence advising performance of biopsies in certain donor types or use of protocol biopsies,2,3 a recent epidemiological study demonstrated that the former only account for 3% of all biopsies performed in Spain every year,4 while only two Spanish centres perform serial protocol biopsies. An analysis of diagnostic biopsies also shows significant differences as regards activity in the different centres, with up to 10-fold differences being found in the reported biopsy/transplant ratios in certain cases.4 In addition, no clear agreement appears to exist either about how renal tissue samples should be obtained and processed. There are relevant differences as regards use of different puncture needle gauges, as well as a quite limited use of more specific diagnostic procedures such as immunofluorescence, immunohistochemistry, or electron microscopy.

Because of this situation, a group of clinicians and nephropathologists experienced in kidney transplant jointly
addressed, in a consensus conference held in Toledo in 2007, the various issues of interest related to the indication,
processing, and evaluation of kidney biopsies in transplant patients. The main conclusions and recommendations on
the subject of such consensus group are summarised in this document.

CLINICAL VALUE OF BIOPSY IN KIDNEY TRANSPLANT

Donor biopsy
Multiple studies have shown that the presence of pre-existing lesions in the donor biopsy is associated to the occurrence of acute rejection, a poorer function, development of subsequent lesions, and a reduced graft survival.5-20 Despite methodological differences between the different studies and certain disparate results,21 there is currently a fair amount of agreement in that the finding of glomerular, tubulointerstitial, and vascular lesions in the biopsy at the time of implant represents one of the main donor-dependent factors that may condition kidney graft evolution. Indeed, it has been shown in several series that the finding in the donor biopsy of ~20% glomerulosclerosis is associated to the presence of delayed graft function (DGF) immediately after transplant, and also to a reduced kidney function or long-term graft loss.8,13-15,18 The presence of tubulointerstitial damage inherited from the donor, such as interstitial fibrosis, tubular atrophy, or acute tubular necrosis, also has a relevant role and is correlated to subsequent development in the recipient of glomerulosclerosis and chronic interstitial damage. Such events significantly influence kidney function and survival of the implanted organ18-20 and emphasise the importance of indication for performance of this biopsy in the donor using a good analytical methodology.22

As regards vascular damage, Nankivell et al found in 2001 that small vessel lesions in the donor predicted for detection of typical chronic nephropathy lesion in biopsies taken at 3 months of transplant,17 whereas in the long term, detection of fibrous intimal thickening in the biopsy at the time of implant has been associated to the occurrence and severity of interstitial fibrosis, as well as to a reduced creatinine clearance.12 Based on all the foregoing, it is understandable that interest in performance of donor biopsies has significantly increased in recent years, in parallel to the increasing use of marginal organs from elderly, diabetic, or hypertensive patients. The presence of these factors is obviously associated to development in the kidney of the abovementioned histological lesions,23,24 which are consistently related to poorer graft evolution outcomes,25,26 as already discussed.

Histological evaluation of the potential graft thus represents, together with clinical and laboratory data, a procedure
that has been used to decide whether a single or double transplant should be performed or organs from that donor should be discarded.27-29 This is the main current indication for donor biopsy in many centres, but it should not
be forgotten that taking a sample at time zero, even in optimum donors, is of great help for adequate interpretation by pathologists of potential subsequent lesions in the recipient. Biopsy should therefore not be limited to the study of graft viability.

Finally, it should also be reminded that biopsy performance in donors is of particular interest in the setting of clinical
trials in order to have paired morphological data before and after transplant.

Recipient biopsy
Histological examination of renal biopsy continues to be the test par excellence for diagnostic identification of graft
pathology, and its value is shown by the fact that more than 90% of biopsies performed in kidney transplant patients in Spain are indicated for diagnostic reasons.4 The main value of biopsy lies in the possibility of differentiating in detail the presence of lesions suggesting acute rejection, nephrotoxicity induced by calcineurin inhibitors, or chronic lesions, and also of diagnosing not clinically suspected conditions such as de novo glomerulopathies, disease recurrence, or nephropathy associated to polyomavirus type BK. In this regard, introduction in recent years of the Banff criteria and their respective updates30-33 has allowed for having a reproducible tool34 with a high clinical-pathological correlation35 and internationally accepted for description of acute and chronic lesions of the different kidney compartments. In addition, recent incorporation of new histological
and marker techniques, such as detection of C4d and donor-specific antibodies, has increased the diagnostic and
prognostic capacity of renal biopsy, giving pathologists the possibility of performing increasingly accurate diagnoses. In this regard, the growing volume of evidence accumulated using these procedures recently allowed the Banff conference for proposing introduction of a new concept, late antibody-mediated rejection, as well as criteria required for its diagnosis.33

With regard to protocol biopsies, recent studies have been able to establish the natural history of lesions affecting the renal graft,36 as well as the impact of each of them on transplant prognosis. Use of serial biopsies has revealed that prevalence of interstitial infiltrates and tubulitis in stable grafts, the so-called subclinical rejection (SCR), is maximum during the initial months after transplant, after which prevalence gradually decreases (though SCR may persist for longer than one year in some patients).38 Presence of these acute lesion, even in cases with minimal inflammation, is consistently associated to the occurrence of chronic lesions such as interstitial fibrosis and tubular atrophy (IF/TA)36,39-41 and to a decrease in long-term graft survival, as recently seen in patients with SCR lesions two weeks after transplant followed up for 10 years.42 Once established, IF/TA progresses rapidly during the first months, and despite being clinically silent at the start,36,43,44 it is also related to a poorer kidney function and to
long-term graft loss45,46 even when detected early at 6 months of transplant.47 In fact, the predictive value of IF/TA is independent from other classical markers such as serum creatinine, proteinuria, or acute rejection.48

In this setting, there has been recently an increasing interest in detection of lesion patterns in biopsies that would help predict transplant evolution better and earlier than separate lesion assessment. The combination of IF/TA and vascular damage has been shown to be significantly associated to a poorer 10-year graft survival as compared to IF/AT alone,49 and similar results were seen at 5 years in patients with IF/TA lesions and transplant glomerulopathy.50 Moreover, the combination of acute and chronic lesions also predicts for poorer long-term results, so that IF/TA has a particularly poorer prognosis when it is concomitantly associated to the presence of SCR.41,50,51 In fact, certain lesion patterns found in protocol biopsies within 6 months of transplant, such as the presence of IF/TA plus transplant vasculopathy or IF/TA plus SCR, have shown no less sensitivity and specificity for predicting 7-year graft viability than classical markers such as acute rejection or kidney function.3

It is also particular interesting to note that protocol biopsies could have a significant role for improving graft prognosis. There are two reports on this regard that, despite their methodological limitations, support this statement.

The first report refers to a study where protocol biopsies were performed 1, 2, 3, 6, and 12 months after transplant.
In this study, Rush et al found that detection and early treatment of SCR with corticosteroid boluses resulted in a
decreased progression of lesions at 6 months and a better kidney function at 2 years as compared to the control
group.52 In the second study, Buehrig et al reported that early detection of nephropathy by the polyomavirus using
protocol biopsies within one year of transplant allowed for modifying immunosuppression in early disease stages. At 6 months, graft survival and function in these patients were significantly superior to patients in whom the disease was detected late when a diagnostic biopsy was performed for kidney function impairment.53

In conclusion, the biopsy not only represents the best alternative for diagnostic evaluation of renal graft lesions, but is also a good tool for prognostic and viability assessment of the graft. Based on the available evidence, it has been stated that protocol biopsies could be considered as a surrogate marker for graft survival,54 and several groups of
researchers are currently focusing their efforts on the search for different lesion patterns, as well as new quantitative parameters for assessing these biopsies, that would improve their predictive value.3 Finally, the significant value of protocol biopsies as secondary endpoint in the setting of clinical trials should not be forgotten. A remarkable experience in this field has already been obtained in several studies intended to describe potential differences in the impact of the different immunosuppressive treatments on renal histology.55-58

RECOMMENDATIONS FOR PERFORMING DONOR BIOPSIES (table I)

Indications

Definite indication
In recent years, the growing disparity between the number of patients in a waiting list for receiving a kidney transplant and the number of grafts available has prompted the need for expanding the criteria for considering a kidney adequate to be transplanted (59). In 2001, the UNOS (United Network for Organ Sharing) defined expanded criteria donors (ECD&