The CT severity index was introduced in 1994 for assessment of the severity of acute pancreatitis  and was accepted internationally but had some limitations [4, 11,12,13,14] including the assessment of extrapancreatic parenchymal complications, the presence of organ failure [4, 12], and peripancreatic vascular complications . Another limitation was the lack of significant difference in morbidity and mortality between patients who have 30–50% pancreatic necrosis and those who have ˃ 50% necrosis. Owing to these limitations, a simplified modified CT severity index offered by Koenraad et al.  was adopted to overcome these limitations.
Ishikawa et al.  was the first to grade the severity of acute pancreatitis according to its retroperitoneal spread using the concept of interfascial plane extension and the uniqueness in Ishikawa et al. ’s study that they found a passageway of spread beyond the interfascial planes, thus succeeded in categorizing all forms of fluid collection in the retroperitoneum into 5 grades.
Both classification systems utilize the natural spread of the disease and recognize the severity and the prognosis of the disease better than the CTSI because it is not accurate enough to represent the location of the retroperitoneal lesions as it is based on the old theory of the retroperitoneal structure .
In our study, we tried to correlate both grading systems and we found that modified CTSI requires intravenous (IV) contrast injection to define pancreatic necrosis and extrapancreatic complications like vascular complications which indeed important prognostic factors that is why we excluded patients with contraindication to contrast administration. Therefore, we searched for another prognostic indicator that does not depend on contrast injection as a complement to the modified CTSI. We applied Ishikawa et al.  grading system for the study group patients, and we faced no difficulty in assessing the extension of inflammatory fluid.
The correlation between the two grading system reveals a significant correlation in detecting morbidity related to the disease in mild pancreatitis (scores 0–2) that was correlated to grades I and II and was considered as a mild disease, while 4 patients with moderate pancreatitis (score 4) were graded as grades I or II; thus, score 4 can be considered as early moderate pancreatitis as these cases resolved spontaneously after medical treatment. Both grading systems show a strong correlation in terms of mortality in the moderate and severe pancreatitis (scores 8–10) that was comparable to the combined grades (IV and V) and was considered as a severe disease. Both grading systems show a good correlation in patients with modified CTSI of moderate pancreatitis (scores 4–6) and with grade III and IV pancreatitis in terms of morbidity and mortality, and this was considered as a moderate disease in agreement with the results of [7, 15] in each grading system.
Both grading systems show a statistical significant correlation in terms of the length of hospital stay, the need for surgical or percutaneous interventions, and the development of organ failure in agreement with the results of [7, 15] in each grading system.
We think that we can apply Ishikawa et al. ’s grading system as a prognostic indicator if there is a contraindication to contrast administration as it does not depend on contrast-enhanced CT.
In the present study, fluid collection confined to the interfascial planes, corresponding to (grade I, II, or III) pancreatitis, resolved spontaneously in all cases (except for one patient with grade II), while inflammatory fluid collection in 3 patients with grade IV disease and in 5 patients with grade V disease needed drainage in agreement with Ishikawa et al.  who mentioned that the fluid in the interfascial planes may drain spontaneously while for grades IV and V, the persistent fluid collection can easily progress to infectious abscess requiring drainage.
In the current study, we did not analyze the ventral extension of pancreatic inflammatory fluid, e.g., into the transverse mesocolon or the lesser sac, and this can be explained by that the fluid collection in these locations often observed in every grade as this fluid results from a direct extension or leakage from the anterior pararenal space in agreement with [6, 7, 18,19,20] studies who reported that local complication, e.g., pseudocyst or pancreatic abscess can develop in these locations with no definite established relation between retroperitoneal grading system and the degree of ventral extension of acute pancreatitis or pancreatic necrosis.
In the current study, we overcome this limitation to some extent by applying modified CTSI as extrapancreatic complications can successfully be recorded as well as peripancreatic collection; thus, we recommend a combined grading system evaluation whenever possible.
Evaluation of the patients by the same radiologist at the same setting in this study decreased the bias; thus, the inter-observer agreement in assessing the severity of pancreatitis was an excellent agreement according to Fleiss et al. ’s criteria.
This study had some important limitations. First, it was done only with contrast-enhanced CT that represents bias as many patients with acute pancreatitis were excluded from the study due to renal impairment, but our aim was to standardize the protocol in both grading systems to know the outcome. Second, it has a small sample size owing to narrow selection criteria. This can be explained by that we performed CT within 1–3 days after the onset of the symptoms aiming to establish the correct prognostic value of both grading systems in acute pancreatitis at the time of presentation, and this leads to a small number of patients and narrow selection criteria because not all patients with acute pancreatitis need to have a CT examination.
To our limited knowledge, this is the first work to correlate combined modified CTSI and interfacial plane grading in the grading of acute pancreatitis and we think that a larger number of patients in multicenter trials need to be conducted for further assessment.