Calculating difference between records vice groups

[pharmIT]

Crystal 8.5 User

I have a subreport that contains various lab values. I only want the most recent lab to be displayed, so I have them grouped by labcode with fields in the group header and the details and group footer suppressed.
I have hit a wall trying to now display the difference between the latest lab for a specific labcode and the previous value. With the formula field in the group, the report returns the difference between groups; ie. 'BUN' - 'CREAT' vice 'BUN' - previous 'BUN'.

Can anyone offer a suggestion on how to fix this? I have included the code I am trying to use. I think it would work within the Detail section, but then I will get more than the latest lab displaying. Thanks in advance!

if {VISITLAB.CODE} = 'BUN' then
{VISITLAB.LABVALUE} - previous({VISITLAB.LABVALUE})
else
if {VISITLAB.CODE} = 'CREAT' then
{VISITLAB.LABVALUE} - previous({VISITLAB.LABVALUE})
else
if {VISITLAB.CODE} = 'HCS_CRCL' then
{VISITLAB.LABVALUE} - previous({VISITLAB.LABVALUE})
else
if {VISITLAB.CODE} = 'WBC' then
{VISITLAB.LABVALUE} - previous({VISITLAB.LABVALUE})

 

[lbass]

First of all your formula should have been using "next" if you are placing it in the group header (with records sorted by descending date) and you don't need the if then because you have already grouped on lab code--but it still won't work with suppressed records.

Another approach would be to add the same table again, linking on lab code and on lab date, using a > link for the lab date FROM the original table TO the visitlab_1 table. Then you can use a formula like this in the group header:

{visitlab.labvalue}-{visitlab_1.labvalue}

-LB