Life Science group revenue decreased 16.6% on a currency neutral basis. Sales of the clinical diagnostics group in the first quarter were $369 million compared to $352 million in Q1 of 2023, which is an increase of 4.7% on a reported basis and 4.8% on a currency neutral basis. Growth of the clinical diagnostics group was primarily driven by increased demand for quality controls, blood typing and diabetes on a geographic basis. Currency neutral year over year revenue for the diagnostics group posted balanced growth across all three regions for the company. Q1 reported GAAP gross margin of 53.4% as compared to 53.5% in the first quarter of 2023 was in line with our expectations as we maintained a tight focus on manufacturing costs, which was partially offset by higher material cost and lower absorption.
Amortization related to prior acquisitions recorded in cost of goods sold was approximately $4 million in both periods. SG&A expenses for Q1 2024 were $215 million, or 35.2% of sales compared to $226 million, or 33% in Q1 of 2023. The decrease in SG&A spend was driven by the positive impact of our previously discussed cost reduction initiatives, including lower employee related expenses and discretionary spend as well as higher restructuring charges in the year ago period. Total amortization expense related to acquisitions recorded in SG&A for the quarter was approximately $1 million for versus approximately $2 million in Q1 of 2023. Research and development expense in the first quarter was $66 million, or 10.9% of sales compared to $75 million, or 11.1% of sales in Q1 of 2023.
Year-over-year decrease was primarily due to decreased employee related expenses and lower restructuring costs. Q1 operating income was $45 million, or 7.3% of sales compared to $62 million, or 9.1% of sales in Q1 of 2023, primarily due to lower sales versus the year ago period, which were partially offset by our expense management initiatives. Looking below the operating line, the change in fair market value of equity security holdings, which are substantially related to Bio-Rad’s ownership of Sartorius AG shares, added $422 million of income to the reported results during the quarter. Interest and other income resulted in net other income of $24 million compared to net other income of $40 million last year. The primary driver of the year over year change is the lower Sartorius dividend, which declined to $18 million in Q1 of 2024 versus the quarter of 2023.
The effective tax rate for the first quarter of 2024 was 21.8% compared to 18.7% the same period in 2023. The effective tax rate reported in these periods was primarily affected by the accounting treatment of our equity securities. First quarter reported net income was $384 million or $13.45 diluted earnings per share compared to net income of $69 million, or a diluted earnings per share of $2.32 in Q1 of 2023. This change from last year is largely related to changes in the valuation of our Sartorius holdings. Moving on to the non-GAAP results looking at the results on a non-GAAP basis, we have excluded certain atypical and unique items that impacted both the gross and operating margins as well as other income. These items are detailed in the reconciliation table in the press release.
Looking at the non-GAAP results for the first quarter in cost of goods sold, we have excluded approximately $4 million of amortization of purchased intangibles and approximately $1 million of restructuring expense. These exclusions moved the non-GAAP gross margin to 54.2% for the first quarter of 2024, which is flat to Q1 of 2023 non-GAAP SG&A dollar spend was slightly lower on a year over year basis, but as a percentage of sales was higher due to lower revenue in Q 124. Specifically, in the first quarter of 24, SG&A as a percent was 34% versus 31.3% in Q1 of 2023 in SG&A on a non-GAAP basis, we have excluded amortization of intangibles of approximately $1 million, approximately $2 billion for an in vitro diagnostic registration fee in Europe for previously approved products and approximately $4 million of restructuring related expenses.
Non-GAAP SG&A as a percentage of sales in the first quarter of 2024 was 10.5% versus 10.4% in Q1 of 2023 in R&D. On a non-GAAP basis, we have excluded approximately $2 million of restructuring expenses and a small acquisition expense. The cumulative sum of these non-GAAP adjustments result in moving the quarterly operating margin from 7.3% on a GAAP basis to 9.7% on a non-GAAP basis. This non-GAAP operating margin compares to non-GAAP operating margin of 12.4% in Q1 of 2023. We’ve also excluded certain items below the operating line, which is primarily related to the increase in value of the Sartorius equity securities and loan receivable holdings of $422 million. The non-GAAP effective tax rate for the first quarter of 2024 was 22.3% compared to 20.9% for the same period in 2023.
A higher rate in 2024 was driven by geographical mix of earnings and change in valuation allowance related to our deferred tax assets. Finally, non-GAAP net income for the first quarter of 2024 was $65 million, or $2.29 diluted earnings per share compared to $99 million, or a diluted earnings per share, $3.34in Q1 of 2023. Moving on to the balance sheet, total cash and short term investments at the end of Q1 2024 was $1,651,000,000 compared to $1,613,000,000 at the end of 2023. The change in cash and short term investments from the fourth quarter of 2023 was primarily due to the change in working capital. Inventory of $783 million was essentially flat compared to $781 million in the prior quarter. For the first quarter of 2024, net cash generated from operating activities was $70 million, which compares to $98 million in Q1 of 2023.
Net capital expenditures for the first quarter of 2024 were $40 million and depreciation and amortization was $37 million. Adjusted EBITDA for the first quarter of 2024, $109 million, or 17.8% of sales and excluding the Sartorius dividend, was 14.8%. The adjusted EBITDA for the first quarter of 2023 was $149 million, or 21.9% of sales and excluding the Sartorius dividend, was 16.8%. During the first quarter, we purchased 14,250 shares of our stock for a total cost of approximately $5 million, or an average purchase price of approximately $330 per share. We continue to be opportunistic with our buyback program and still have approximately $275 million available for share repurchases under the current board authorized program. Moving on to the non-GAAP guidance as referenced in Andy’s commentary, we are seeing some encouraging signs in life science end markets.
However, we remain cautious on the magnitude and timing of the recovery for the life Science group, but are still anticipating improvement during the second half of the year. We continue to expect normalized growth for the clinical diagnostics group in 2024. Taken together, we are maintaining our full year outlook with currency neutral revenue growth to to be between one and 2.5% and non-GAAP operating margin projected to be between 13.5% and 14%. I’ll now hand the call back to Norman to make a few concluding remarks.
Norman Schwartz: Thanks Roop. Just to close it out, I’d like to reiterate that in spite of all that’s going on around us, our strategy and our focus for the future growth of the company is intact. In our clinical diagnostics business, we have these leading market positions globally for our core platforms and we continue to invest in supporting the growth and building a position in, for example, the new molecular diagnostic segments through the development of PCR one, an acquisition we made some time ago, and leveraging our droplet digital PCR platform into high value niches in life science, we continue to maintain a focus on biopharma, especially for digital PCR. Our process chromatography products and new products in development say particularly around cell biology.
