Second‐Knee C‐Reactive Protein After 1‐Week Staged Bilateral Total Knee Arthroplasty

ABSTRACT Background One‐week staged bilateral total knee arthroplasty (TKA) operates the second knee before the systemic response to the first has resolved. Whether a higher second‐knee C‐reactive protein (CRP) reflects expected carryover or a distinct second‐surgery response is unknown. Using each patient as their own control, we assessed how far this burden reflects carryover. Methods We retrospectively analyzed 216 patients who underwent 1‐week staged bilateral TKA between January 2020 and April 2026 (primary cohort: 210 staged within 9 days). Primary endpoints were the absolute CRP on Postoperative Days 2 and 5, with the Days 2–5 area under the curve as a complementary summary. Exploratory analyses examined the per‐surgery CRP increment from each operation's own floor, peak reproducibility, and operative‐magnitude adjustment. Paired comparisons used the Wilcoxon signed‐rank test. Results The second knee carried a higher absolute CRP burden than the first: median paired differences +1.44 mg/dL at Day 2, +0.70 at Day 5, and +3.74 mg · day/dL for the Days 2–5 area under the curve, all significant after Holm correction and robust to multiple imputation of missing values. From each operation's own floor, the second‐knee increment was smaller, but the Day‐5 proxy floor exceeded the directly measured pre‐second‐operation value by a median 2.58 mg/dL, underestimating that increment by the same amount. In the 14 patients with a measured floor and a complete increment pair, the increment difference was −0.55 mg/dL and did not reach significance ( p = 0.15); the measured floor was itself of similar magnitude to the Day‐2 between‐knee difference. Peak CRP showed moderate within‐patient agreement (intraclass correlation 0.65). Conclusions In 1‐week staged bilateral TKA without tranexamic acid or routine corticosteroid, the second knee runs a higher absolute CRP course, consistent with substantial carryover from the first operation; the data cannot exclude an additional per‐surgery response or fix its share. Two periprosthetic joint infections occurred, one during the index admission and one at 48 months, and CRP was not tested against infection outcomes, so these data support no diagnostic threshold. A modestly higher second‐knee CRP may reflect the expected acute‐phase course but must not override clinical suspicion; prospective validation is needed.