Review for 10.1186/s12876-022-02588-0 (2022)

Reviewer’s report
Title: 
No association of serum ferritin levels with advanced liver fibrosis in
untreated German patients with autoimmune hepatitis
Version: 
1 
Date: 
19 Aug 2022
Reviewer's report:
The authors incorporated most of the suggested modifications, however there are
still a few issues that need to be corrected.
1) The authors stated in their answer: "According to our biometric counselling,
that we performed during the preparation of the Letter we were instructed that a
validation of results does not implicate to start a new modelling (meaning a new
logistic regression analysis) but rather to apply the cut-off identified in the
training cohort (the Chinese cohort from Chen et al.). Therefore, we applied the
ferritin cut-off of 199 µg/ml and found no significant association of a ferritin
level above 199 µg/l with advanced fibrosis in our German cohort." 
I think there is a misunderstanding here. It is true that when there is a defined
cut-off level, the authors need to validate that cut-off during the validation
process and not define a new one in their cohort. Therefore, the comparison part
of the present study is correctly done. However, in the logistic regression
analysis, Chen et al used ferritin level as a continuous variable (dichotomisation
by the cut-off level of 199 µg/L happened later). If one performs univariate
logistic regression on ferritin levels and include the same factors in the
multivariate analysis that was reported in the original article that is not a new
modelling but the only way to validate the odds ratios reported in the original
study. It does not matter how little is the likelihood of a statistically significant
result it still needs to be shown. (Of note, lack of statistical significance in case
of the univariate analysis is enough to disprove the previous finding and no
further multivariate analysis is needed). However, I agree that it most probably
will not change the conclusion of the paper, since the results of both the ROC
analysis and the comparison by the cut-off level were not statistically significant
in contrast to what Chen et al reported.
2) What did the authors mean by "The odds ratio for advanced fibrosis using
absolute
ferritin levels with a cut-off of 199 
μ
g/l as published by Chen et al. was 1.444
(95% CI 0.571 -
3.653)." Did the authors perform logistic regression using the cut-off value
instead of using ferritin as a continuous variable? Why did they find this more
appropriate? Please be specific when explaining the performed analyses in the
text!
3) The authors misinterpreted the reason for logarithmic transformation.
Logarithmic transformation is not necessary for a simple comparative test, since
methods to compare not normally distributed populations exist (Mann-Whitney)
that the authors also used (table 1). Logarithmic transformation was suggested as
a possible tool to increase the odds ratio of the regression analysis (since it
showed only a marginal effect in the article from Chen at al).
4) I assume, instead of "compensated for by using binary logistic regression and
multivariate regression analysis" (top of page 3), the authors wanted to write
"corrected by including these parameters in the univariate and multivariate
binary logistic regression analyses".
5) There is another confusion around the sentence "Both approaches have their
pros and cons which is why we presented both." 
When I wrote it in my comments, I was referring to the sentence quoted in point
4). I.e.: approach one - including the interfering parameters in the univariate and
multivariate regression analyses; and approach two: correcting for the upper
limit of normal. However, the authors, as it was discussed above, did not present
the results of the univariate and multivariate regression analyses using ferritin as
a continuous variable like Chen et al did.
Considering the immediately preceding sentence the authors probably meant that
they performed comparison between fibrotic and non-fibrotic patients and also
carried out ROC analysis using both absolute and corrected ferritin levels. But
this is also confusing without explaining what the mentioned pros and cons are in
this case. Furthermore, the authors also mentioned the logarithmic
transformation which made it even less clear what they were talking about.
I suggest that the logarithmically transformed ferritin level be omitted from the
comparative analysis, and instead of the quoted sentence, simply state that the
authors believe that the results obtained using ULN-corrected ferritin levels are
more valid than the results obtained using the absolute values, but both is
presented for fair validation.
6) The manuscript contains several grammatical errors. Please perform a
grammar check!
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Is the study design appropriate to answer the research question (including the use
of appropriate controls), and are the conclusions supported by the evidence
presented?
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from apparent manipulation?
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