Background:

Alzheimer’s disease cerebrospinal fluid (CSF) biomarkers amyloid-β 1–42 (Aβ₄₂), total tau (T-tau), and phosphorylated tau 181 (P-tau₁₈₁) are widely used. However, concentration gradient of these biomarkers between intraventricular (V-CSF) and lumbar CSF (L-CSF) has been demonstrated in idiopathic normal pressure hydrocephalus (iNPH), potentially affecting clinical utility.

Objective:

Here we aim to provide conversion factors for clinical and research use between V-CSF and L-CSF.

Methods:

Altogether 138 iNPH patients participated. L-CSF samples were obtained prior to shunt surgery. Intraoperative V-CSF samples were obtained from 97 patients. Post-operative follow-up L- and V-CSF (shunt reservoir) samples of 41 patients were obtained 1–73 months after surgery and then after 3, 6, and 18 months. CSF concentrations of Aβ₄₂, T-tau, and P-tau₁₈₁ were analyzed using commercial ELISA assays.

Results:

Preoperative L-CSF Aβ₄₂, T-tau, and P-tau₁₈₁ correlated to intraoperative V-CSF (ρ= 0.34–0.55, p < 0.001). Strong correlations were seen between postoperative L- and V-CSF for all biomarkers in every follow-up sampling point (ρs Aβ₄₂: 0.77–0.88, T-tau: 0.91–0.94, P-tau₁₈₁: 0.94–0.96, p < 0.0001). Regression equations were determined for intraoperative V- and preoperative L-CSF (Aβ₄₂: V-CSF = 185+0.34*L-CSF, T-tau: Ln(V-CSF) = 3.11+0.49*Ln(L-CSF), P-tau₁₈₁: V-CSF = 8.2+0.51*L-CSF), and for postoperative V- and L-CSF (Aβ₄₂: V-CSF = 86.7+0.75*L-CSF, T-tau: V-CSF = 86.9+0.62*L-CSF, P-tau₁₈₁: V-CSF = 2.6+0.74*L-CSF).

Conclusion:

Aβ₄₂, T-tau, and P-tau₁₈₁ correlate linearly in-between V- and L-CSF, even stronger after CSF shunt surgery. Equations presented here, provide a novel tool to use V-CSF for diagnostic and prognostic entities relying on the L-CSF concentrations and can be applicable to clinical use when L-CSF samples are not available or less invasively obtained shunt reservoir samples should be interpreted.