One option using interior titled code
to first draw the gray filled rounded rectangle for the title and then the rounded rectangle for the contents:
\documentclass{book}
\usepackage[many]{tcolorbox}
\usetikzlibrary{calc}
\definecolor{mp}{RGB}{240,240,240} % Farbe für Nummerierung
\tcbset{ % Rahmen
arc=20pt,
outer arc=20pt,
title=Wert,
coltitle=black,
enhanced,
freelance,
frame code={},
interior titled code={
\fill[mp,rounded corners=12pt]
([xshift=-1pt,yshift=-10pt]interior.west)
rectangle
([xshift=50pt,yshift=3pt]title.north west);
\draw[rounded corners=20pt,color=mp, line width=1pt,fill=white]
(frame.south west) --
(title.south west) --
(title.south east) --
(frame.south east) -- cycle;
},
}
\begin{document}
\begin{tcolorbox}
Erwachsen \\
8,9 - 10,1 mg/dL
2,25 - 2,75 mmol/L
Kinder höher
\end{tcolorbox}
\end{document}

The definition for the \wert
command then might be something like
\documentclass{book}
\usepackage[many]{tcolorbox}
\usetikzlibrary{calc}
\definecolor{mp}{RGB}{240,240,240} % Farbe für Nummerierung
\newcommand\wert[1]{%
\begin{tcolorbox}[
arc=20pt,
outer arc=20pt,
title=Wert,
coltitle=black,
enhanced,
freelance,
frame code={},
interior titled code={
\fill[mp,rounded corners=12pt]
([xshift=-1pt,yshift=-10pt]interior.west)
rectangle
([xshift=50pt,yshift=3pt]title.north west);
\draw[rounded corners=20pt,color=mp, line width=1pt,fill=white]
(frame.south west) --
(title.south west) --
(title.south east) --
(frame.south east) -- cycle;
}
]
#1
\end{tcolorbox}%
}
\begin{document}
\wert{Erwachsen \\
8,9 - 10,1 mg/dL
2,25 - 2,75 mmol/L
Kinder höher}
\end{document}
If the title for the boxes must be variable, a simple modification for the above code (introducing an optional or mandatory argument for the tcolorbox
) will suffice.