Keep values in data frame= Na (sodium in chemistry) as is

Original df (clinical chemistry)

Subject Code Test Value Units   Flag
1       NA    NA   147   mmol/L    
2       NA/K  NA/K 10.5  RATIO  
3       K     K    4.7   mmol/L 
4       CK    CK   235   UL
...

Ideal df after cleaning

Subject Code  Test             Value  Units   Flag
1       NA    Sodium           147   mmol/L   NA
2       NA/K  Sodium Potassium 10.5  RATIO    NA
3       K     Potassium        4.7   mmol/L   NA
4       CK    Creatine Kinase  235    UL      NA
...

What I have tried

df <- read.csv(file="clinchemistry.csv", header = TRUE, sep=",", stringsAsFactors = FALSE)

df$df[df8$Test == "NA"] <- "Sodium"

df$df[df8$Code == "NA"] <- "Sodium"

and

df[is.na(lb$Code)]<-"Sodium"

lb[is.na(lb$Code)]<-"Sodium"

RESULTS:

All the sodium values disappear or get an error:

Error in [<-.data.frame(*tmp*, is.na(lb$Tesst), value = "Sodium") : duplicate subscripts for columns

WOULD SOMEONE GUIDE MY THINKING?