[Tex/LaTex] Automatic line break inside multicolumn table

multicolumntables

I'm trying to create a table as shown below. I'm not sure why my code makes the "Sample ID" column as wide as the "Composition" column even though they have a different setting in the multicolumn{}{this}{} line. Is there a way to control the width in an easier way or am I just too blind to see the mistake I made somewhere?

\begin{tabular}{|p{1.5cm}|p{3.5cm}|p{6cm}|p{1cm}|} 
\multicolumn{2}{p{5cm}}{\textbf{Date:}} & \multicolumn{1}{c}{\textbf{Donor ID:}} & \multicolumn{1}{p{1cm}}{} \\
\multicolumn{2}{p{5cm}}{\textbf{ }} & 
\multicolumn{2}{p{7cm}}{\textbf{ }}\\
\multicolumn{2}{p{5cm}}{\textbf{Serial index:}} & \multicolumn{2}{p{7cm}}{\textbf{ }} \\
\multicolumn{2}{p{5cm}}{\textbf{ }} & 
\multicolumn{2}{p{7cm}}{\textbf{ }}\\
\hline 
\textbf{Solution} & \textbf{Sample ID} & \textbf{Composition} & 
\textbf{Init} \\ \hline 
1 & \underbar{WB - Human Whole Blood} & \multicolumn{1}{|p{6cm}|}{The WB is gently mixed in the vacutainer and poured into a plastic beaker before it is collected in an appropriate sized syringe} &  \\ \hline 
2 & \underbar{Ref WB} & \multicolumn{1}{|p{6cm}|}{Approx. 1 mL HWB is collected for reference measurements on the flow cytometer and an ABL90} & \\ \hline 
\end{tabular}

Update:
The code is updated due to a fine adjustment that was necessary. A new image is uploaded as well to show the problem. I want cell (2,2) to wrap the text, but i doesn't seem like \underline allows that operation? any suggestions?

Original situation

New situation

Best Answer

edit:

like this (considering your comment below answer)?

enter image description here

it is obtained by use of tabularx table environment, macros \thead and \makegapedcells from makcell package and appropriate selection of columns type. for underlined text in second column's cells is used \ul{...} instead of \underbar{...} (which not enable to break lines) from package soul which enales to brak text in cells. for values with measure i suggest to use \siunitx package:

\documentclass{article}
\usepackage{geometry}
\usepackage{makecell, tabularx}
\renewcommand\theadfont{\normalsize\bfseries}
\renewcommand\theadgape{}
\setcellgapes{3pt}
\newcommand\mcl[1]{\multicolumn{2}{l}{\textbf{#1}}}
\usepackage{soul}    % <---
\usepackage{siunitx} % <---

%---------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%

    \begin{document}
    \begin{table}[htb]
\makegapedcells
\begin{tabularx}{\linewidth}{|c|l|X|c|}
\mcl{Date:}         &   \mcl{Donor ID:}                             \\[2ex]
\mcl{Serial index:} &   \mcl{}                                      \\[2ex]
    \hline
\thead{Solution}    & \thead{Sample ID}
                            & \thead{Composition}   & \thead{Init}  \\
    \hline
1   & \ul{WB - Human Whole Blood} % <---
                & The WB is gently mixed in the vacutainer and poured into a plastic beaker before it is collected in an appropriate sized syringe
                            &                                       \\
    \hline
2   & \ul{Ref WB} 
                & Approx. \SI{1}{mL} HWB is collected for reference measurements on the flow cytometer and an ABL90
                            &                                       \\
\hline
3   & \ul{WB - Human Whole Blood and more stuff}
                & Approx. \SI{1}{mL} % <---
                  HWB is collected for reference measurements on the flow cytometer and an ABL90
                            &                                       \\
   \hline
\end{tabularx}
    \end{table}
\end{document}
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