Introduction Lumbar puncture (LP) is a widely used diagnostic method in patients of all ages. Blood-contaminated cerebrospinal fluid samples are frequent and may compromise diagnostic accuracy.

Objectives We determined age-specific incidences of traumatic LPs (TLPs) in adults and examined factors that accounted for the incidence of TLPs.

Methods Erythrocyte count data from 15,812 LP procedures (2,404 were performed twice) were collected from hospital records of patients aged from 18 to 104 years. The incidence of TLPs in a patient's second LP procedure was evaluated with logistic regression analysis using the first LP, the time between the procedures and age as predictors.

Results The incidence of TLP in the second procedure was at least double that in the first procedure. If the first procedure was traumatic, the odds ratio of a TLP in the second procedure was 7–40-fold. One day between the successive procedures was associated with an over 10-fold odds ratio increase of TLP, and a week was still 4–8-fold odds ratio increase. Age was also associated with the incidence of TLP.

Conclusions Two factors (a week or less between a patient's two LP procedures or a traumatic first LP) multiply the odds of the second procedure being traumatic and contribute to whether a patient's following LP procedure is successful.