SUM() 3 columns with alias into a fourth one
You cannot refer to an alias defined in the same SELECT list. There are various workarounds. Assuming your query is something like this:

select nome, 
       -- various complex expression / subqueries
       &lt;complex_expression_or_subquery&gt;
           as Laudos_Expedidos,
       &lt;complex_expression_or_subquery&gt;
           as Laudos_Expedidos2
from &lt;table_name&gt; ;



Duplicate code. As this is the least attractive solution, for various reasons, I won't add a code sample. 
a derived table or a CTE allows to use the aliases as they are already defined in a previous level:

with cte as
( select nome, 
         -- various complex expression / subqueries
         &lt;complex_expression_or_subquery&gt;
             as Laudos_Expedidos,
         &lt;complex_expression_or_subquery&gt;
             as Laudos_Expedidos2
  from &lt;table_name&gt;
) 
select c.*,
       c.Laudos_Expedidos + c.Laudos_Expedidos2
           as Total_Laudos_Expedidos
from cte as c ;

Use CROSS / OUTER APPLY allows also to refer to previously defined aliases. This offers the possibility of using multiple outer/cross apply subqueries, if you want to use the aliases of the first apply in further calculations. (With the derived/CTE solution, that is also possible but the code usually looks more complicated):

select t.nome, 
       f.*,
       g.Total_Laudos_Expedidos,
       f.Laudos_Expedidos / g.Total_Laudos_Expedidos
           as Percent_Expedidos
from &lt;table_name&gt; as t
  outer apply
    ( select 
          -- various complex expression / subqueries
          &lt;complex_expression_or_subquery&gt;
              as Laudos_Expedidos,
          &lt;complex_expression_or_subquery&gt;
              as Laudos_Expedidos2
    ) as f 
  outer apply
    ( select 
          f.Laudos_Expedidos + f.Laudos_Expedidos2
              as Total_Laudos_Expedidos   
    ) as g ;