Both cations and anions can be filtered . Ionized drugs can pass through slit pores, i.e., it does not matter whether the drug is a weak acid or weak base, charged or uncharged. Negative charges on the slit pores DO repel proteins, but most drugs are small molecules and charge considerations are not of practical importance for them.
Creatinine clearance is used as an index of glomerular filtration.
Can manipulate the pH to hasten elimination of toxins.