I think it might stand for direkt (direct), see info below:

Why accept a calculated LDL result now that an accurate, measured, Direct LDL test is available?

Given the importance of LDL cholesterol as a focus of coronary heart disease (CHD) risk reduction, it is an irony of contemporary medicine generally not recognized by clinicians that LDL cholesterol values are based on an estimate.

Many laboratories currently use the equation known as the Friedewald Formula to estimate LDL cholesterol concentration. This formula estimates LDL concentration by subtracting the cholesterol associated with other classes of lipoprotiens from total cholesterol. The limitations of the Friedewald estimate are well documented but not widely appreciated.

When LDL cholesterol measurements are ordered, most clinical laboratories default to estimating LDL cholesterol values by means of the Friedewald formula.

Problems associated with the Friedewald (calculated) Method:

The Friedewald Formula requires multiple steps, with each step adding a potential source for error.
The method is innaccurate as triglyceride levels increase.
Patients must fast before the test is administered in order to avoid a triglyceride bias.
http://www.directldl.com/healthcare/dldl_healthcare_calcldl....

http://labtestsonline.org/understanding/analytes/dldl/tab/gl...