In this retrospective case-control study, we attempted to develop a discrimination model used to distinguish patients with PLA2R-negative MN and MCD. we collected and analyzed the basic information and laboratory examination of 949 patients with idiopathic MN or MCD. Based on 200 PLA2R-negative patients and 144 MCD patients, we developed a discrimination model to differentiate the two diseases. The results showed great diagnostic effectiveness with an AUC of 0.889 in training group and an AUC of 0.920 in test group as well as high calibration capability. To the best of our knowledge, it is the first study aiming to develop a discrimination model based on the basic information and the laboratory examination of participants to distinguish primary PLA2R-negative MN and MCD. In addition, our model also showed good diagnostic effectiveness with an AUC of 0.856 in all idiopathic MN patients (either PLA2R-negative or PLA2R-positive MN) and MCD patients. It is an attempt at translational medicine of our study, which can aid clinicians to treat patients with different methods in a timely manner and thus improve their prognosis.
Currently, it is difficult to distinguish MN and MCD patients by a noninvasive tool in clinical practice. A study tried to use soluble urokinase-type plasminogen activator receptor (suPAR) level to distinguish idiopathic focal segmental glomerulosclerosis (FSGS), MN and MCD, However, the study revealed that the three types of glomerulopathy cannot be distinguished using suPAR solely [13]. Therefore, there is no miraculous indicator or model to identify these two primary glomerular diseases currently.
Prediction and discrimination models based on clinical data have been developed increasingly in a wide variety of diseases recent years[14, 15]. In terms of kidney diseases, prediction and discrimination models are also rapidly growing owing to its scientific nature and accuracy[16-18]. The appearance of clinical models gave us great inspiration.
Present evidences suggested that MN had the largest proportion of morbidity in elderly patients, while MCD accounts for the highest proportion of primary nephrotic syndrome in young patients[3, 19, 20]. Our experiments reached similar results that the age at biopsy had a certain influence on the nomogram. In our study, patients with older age were more likely to be considered as PLA2R-negative MN, while younger age at onset was considered to be a higher risk of MCD.
ALB level was one of the predictive factors in this model. Some study showed MN patients always had higher ALB levels, which was consistent with our results[21, 22]. One of the most important clinical manifestations of nephrotic syndrome is increased urinary protein and decreased albumin level. Larger amounts of glomerular albumin filtration will also make serum albumin and serum total protein at a low level. MCD patients always presents as an acute onset, severe illness, and a greater amount of urinary protein, and rapid decline in renal function may result in increased Scr levels and decreased eGFR and 24h urine volume. Some patients will even progresses to AKI,which is relatively common actually,due to high-grade proteinuria[23]. The slit diaphragm between foot processes is regarded as a fine filter[24]. There is a common assumption that proteins leak from the slit pores due to reduced nephrin expression, leading to larger amounts of glomerular albumin filtration in MCD patients[25]. In our study, we chose ALB levels as one of the variables of the model by multivariable regression analysis to exclude the effect of collinearity.
HDL level is another variable that can be used to distinguish between two diseases according to our results. Nephrotic syndrome could cause upregulation of HDL endocytic receptor and downregulation of HDL docking receptor, causing dysregulation of lipid/lipoprotein metabolism[26]. MCD is also known as lipoid nephropathy because steatosis can be observed in epithelial cells of proximal convoluted tubules under light microscopy. In addition, increased hepatic lipoprotein synthesis and reduced lipoprotein degradation are also thought to be responsible for elevated blood lipid profiles. Takeshi Fujita compared lipid and fatty acid metabolism between 7 MCD and 11 MN patients. The results showed that the patients with MCD had higher level of blood lipids than MN[27]. Although the mean HDL level was much higher in patients with MCD, there was no statistical significance between the two groups, which was not exactly the same as our results. The reason might be that their sample size was not large enough, leading to the unobvious statistical significance.
Increased urea level was usually observed in a high protein decomposition status. After the renal filtration barrier disrupted, a large amount of proteins will leak into Bowman’s space and renal tubules through glomerular barrier to form crude urine. When proximal tubules enhance the reabsorption of filtered proteins, the protein decomposition is also increased at the same time, resulting in elevated serum Urea level. Serum Urea level were also found different in MN patients and MCD patients in Jin Dong’s research[28], which was consistent with our results. The Urea level plays an important role in our discrimination model.
Red blood cell count and Hb levels had statistical significance by means of univariate regression in our study. They are usually recognized as the indices to evaluate anemia. Compared with younger patients, idiopathic membranous nephropathy patients over 65 years old were found to have lower Hb level than patients less than 65 years old in Choi JY’s study[29]. However, the results in Yaeni Kim’s study showed there was no difference of Hb levels between elderly patients and young patients[30]. The reason for this ambiguity might be different gender and illness state of included patients. In our study, univariate regression showed there was no statistical difference in gender. And the data we collected was from the time of renal biopsy, reducing influence of the illness state.
The decision curve showed the clinical utility of our model, indicating it may be beneficial for clinicians to distinguish the two diseases by using our model. And using the nomogram to distinguish the two diseases added more benefits than either all or no patients who underwent a renal biopsy if the threshold probability of a patient was > 1%. The results of decision curves suggest the good clinical application value of our model, reflecting the thinking mode of translational medicine.
The results of diagnosis efficiency test in potentially relevant cases suggested that our model is applicable to all idiopathic MN and MCD patients. Some hospitals are unable to perform PLA2R test, and our model might provide an alternative tool for these hospitals to distinguish MN and MCD.
Our study is an attempt in translational medicine and has a number of strengths. It had a large sample size with 949 idiopathic MN and MCD patients confirmed by renal biopsy. And the 5 items in the nomogram are routine clinical variables that can easily obtained by clinicians. We chose to collect the information and examination results at the time of renal biopsy, and excluded the influence of corticosteroid or immunosuppressive agents. What is more, our discrimination model has satisfying diagnostic effectiveness with an AUC of 0.889 in training group and an AUC of 0.920 in test group. The outstanding discrimination ability for all idiopathic MN and MCD patients even showed wider application prospects of our model. The operation of the model is simple and fast, which can help doctors diagnose patients timely. Unlike renal biopsy, our model doesn’t have any contraindications so that it can be used more widely.
However, there are also several limitations in our study. First, we still need to expand the sample size for further reducing the heterogeneity. In addition, all the patients came from the First Affiliated Hospital of Zhengzhou University and we did not conduct multicenter external validation. Thus, we can’t exclude the influence of diet, race and other related factors on the experimental results. Last, our model is only suitable for the identification of MN and MCD. There is still a lack of differential ability of our model for other types of nephrotic syndrome like focal segmental glomerulosclerosis (FSGS), IgA nephropathy (IgAN) and membranoproliferative glomerulonephritis (MPGN). Corrections to these shortcomings will be made in our subsequent research.